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FAQ Fisheries Technical Paper No. 193 FIRM/T193

A REVIEW OF THE WORLD RESOURCES
OF MESOPELAGIC FISH

J. Gjrfsaeter and K. Kawaguchi

University of Bergen University of Tokyo

Bergen, Norway Tokyo, Japan

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS

Rome, 1980

The designations employed and the presentation
of material in this publication do not imply the
expression of any opinion whatsoever on the
part of the Food and Agriculture Organization
of the United Nations concerning the legal
status of any country, territory, city or area or
of its authorities, or concerning the delimitation
of its frontiers or boundaries.

ISBH 92-5^100924-4

The copyright in this book is vested in the Food and Agriculture Orga-
nization of the United Nations. The book may not be reproduced, in whole
or in part, by any method or process, without written permission from
the copyright holder. Applications for such permission, with a statement
of the purpose and extent of the reproduction desired, should be addressed
to the Director, Publications Division, Food and Agriculture Organization
of the United Nations, Via detle Terme di Caracalla, 00100 Rome, Italy.

FAO I960

PREPARATION OF THIS PAPER

This review is part of a continuing effort by the Fishery Resources and Environment
Division to provide current information on the potential and state of exploitation of the
world f s fishery resources, conventional and unconventional. Mesopelagic fish are hardly
exploited at present, and there are a number of practical problems to be solved before they
can be harvested commercially on a large scale, but their biological potential is large.

The authors of this paper collaborated in writing the introductory chapters on general
topics (Ch. 1-4) and the concluding chapters (Ch. 21 and 22). Of the intervening area
reviews, those covering the Atlantic Ocean and the western Indian Ocean were prepared by
Dr. Gjrfsaeter, and the rest by Dr. Kawaguchi.

ACKNOWLEDGEMENT

We wish to thank all colleagues who have contributed data and other information used
in this report.

We also thank J.A, Gulland and W.G. Clark of FAO Fishery Resources and Environment
Division for many useful comments and suggestions to the manuscript, and for helpful dis-
cussions. Our thanks are also due to other staff members, L.K, Boerema, J.-P. Troadec,
S. Chikuni, W. Fischer, G. Sharp and J. Burczynski, who read parts of the manuscript and
offered valuable comments.

Distribution:

FAO Fisheries Department

FAO Regional Fishery Officers

Selector SM

Authors

For bibliographic purposes this document
should be cited as follows:

Gjrfsaeter, J, and K. Kawaguchi, A review of
1980 the world resources of mesopelagic
fish. FAO Fish. Tech. Pap., (193):
151 p.

- VI -

18. SOUTHEAST PACIFIC 11!

19. SOUTHWEST PACIFIC 111

20. THE ANTARCTIC 115

21. DISCUSSION OF FISHERY POTENTIAL 12]

21.1 Summary abundance information 121

21.2 Potential yield 12;

21.3 Species of potential commercial interest 12(
21. A Present catches 13(

22. NEEDS FOR FUTURE STUDIES 13!

23. REFERENCES 131

- 1

1 . INTRODUCTION

During recent years an increasing proportion of conventional fish stocks has reached a
state of full exploitation or even over-exploitation, and the search for new fishery resources
has been intensified. At present, it seems that krill, cephalopods and mesopelagic fish are
the most promising potential resources (Gulland, 1971; Okutani, 1977).

A fish species can be called mesopelagic if it spends the day in the mesopelagic zone.
The mesopelagic zone has been defined in different ways based on depth, temperature and light
regimes. For the present purpose, depth seems to be the best criterion and mesopelagic fish
can thus be defined as species generally spending the day at depths between approximately
200 and 1 000 m. Generally they perform a diel migration, coming to the upper 200 m or even
to the surface during the night.

Species of many fish families will fall within this definition, but generally the Mycto-
phidae and Gonostomatidae are dominant. Sternoptychidae, Bathylagidae, Chiasmodontidae,
Trichiuridae, Nomeidae and others seem to be fairly important in some areas. In this report
the main emphasis will be placed on the Myctophidae and on the mesopelagic members of the
family Gonostomatidae.

Mesopelagic fish are found in all oceans from the Arctic to the Antarctic, but the
number of species and, in general, the annual production are highest in subtropical and trop-
ical seas. Both typically oceanic and neretic species are found.

There are rather few examples of utilization of mesopelagic fish. The most well known
is the purse seine fishery for Lampanyctodea heetoris off South Africa (Newman, 1977), but
the fishing fleet of the U.S.S.R. are also fishing myctophids off West Africa, and of f South-
east Australia mesopelagic fish (Maurolieus muelleri and Lampanyetodes heetoris) have been
caught during exploratory fishing.

Fish meal and oil and fish silage have been produced, but some species (Diaphus coeru-
leus and Gyrmoacopelus nioholski) have also been fished for human consumption (Melnikova,
1973; Dubrovskaya and Makorov, 1969). Other species seem to have adverse effects due to
their high content of wax esters.

The bulk of early work on mesopelagic fish was concerned primarily with taxonomy and
distribution. There are still many unsolved problems in these fields, but the recent lit-
erature has been mostly concerned with vertical distribution and migration and other aspects
of the ecology of the group (e.g. Clark, 1973, 1974; Badcock and Merrett, 1976, 1977; Roe,
1974) . The larvae of Myctophidae and Gonostomatidae have also been intensively studied
during recent years (e.g. Moser and Ahlstrom, 1970, 1972, 1974 and Ahlstrom, 1974).

Knowledge about the food and feeding habits of mesopelagic fish has also increased and
their importance as vertical transporters of organic matter has been stressed (e.g. Legand
and Rivaton, 1969; Merrett and Roe, 1974; Hopkins and Baird, 1977). The literature gives a
fairly clear picture of what mesopelagic fish eat, but it is less informative as to what eats
mesopelagic fish.

Comprehensive data on the life history of any mesopelagic species is sparse. Age and
growth are known for some of the more important cold-water species but only tentative in-
formation is available on tropical ones. The reproductive biology is still little known.

The present report briefly reviews the systematics, ecology and life history of impor-
tant groups of mesopelagic fish. Further, some methods used for abundance estimation of
these fish are described and discussed. The bulk of the report reviews the present knowledge
of abundance and species composition of mesopelagic fish in each of the FAO statistical areas.
Although the data in most cases are few, an attempt has been made to give tentative estimates
of the biomass and production in the various areas. When available, information on the life
history of the most important species in each area is also given.

- 3 -

2 . SYSTEMATICS
2-1 Families of mesopelagic fishes

Beneath the epipelagic zone, fishes belonging to about 100 families have been known to
occur. Of these, 73 families are the so-called "oceanic deep-water fishes 11 and the other
26 are "secondary deep-water fishes' 1 (Rass, 1967). The fishes of the mesopelagic zone are
mostly oceanic deep-water fishes and belong to about 30 families, although members of some
families occur also in the epipelagic and bathypelagic zones (Rass, 1967; Parin, 1968).

The families of mesopelagic fishes are listed in Table 2.1, with the number of genera in
each family, based on Lindberg (1971). The number of families and genera, and their arrange-
ment, are different in the various classification systems (Berg, 1940; Greenwood et dl .* 1966;
Gosline, 1971). However, the aim of this report is not to present a detailed systematic
account. Based on the number of genera in each family, the fishes of the families Gono s to-
mat id ae (20 genera), Melanostomiatidae (15), Myctophidae (30), and Gempylidae (20) are the
most diverse at the generic level. Of these, fishes of the Myctophidae and Gonostomatidae
account for 60 - 90 percent of the total catch of micronektonic fishes, both in weight and
number, and are thus the most important groups from both the taxonomical and ecological
point of view.

2.2 Identification of myctophid fishes

The identification of myctophid fishes has been difficult due to taxonomic confusion.
Recently such confusion has been gradually reduced as some comprehensive keys to species
have become available for some or most of the genera in certain areas. But the amount of
study has been very uneven among areas and genera, and the more recent works have always
complemented the older ones.

Here we shall only refer to comprehensive works presenting good descriptions and keys
to species, which were contributed to journals or books easily available even to people un-
familiar with myctophid taxonomy. Considering the great number of species, of which some
are cosmopolitan and others are endemic to only some areas, we shall deal with this family
ocean by ocean*

Atlantic Ocean: Knowledge of the myctophid fauna is well advanced in the North Atlantic
Ocean. For identification of the 82 North Atlantic species, the description and keys by
Nafpaktitis et al. 3 (1977) are the most comprehensive. Kreft and Bekker (1973) have prepared
a checklist of 57 species in the eastern North Atlantic, north of 30N, and the Mediterranean.
All of these 57 species are included in the keys of Nafpaktitis et al. 3 (1977), with the ex-
ception of one species, Ctenosoopelus phengodes (Liitken, 1892) , whose occurrence is based on
a single specimen reported by Zugmayer (1911) and is doubtful. The South Atlantic Ocean
shares many species with the North Atlantic (Nafpaktitis et al., 1977). However, a comparison
of the species lists of the North and South Atlantic (Nafpaktitis et al., 1977; Parin et al. 3
1974; Kreft, 1974) suggests that at least 30 species of 14 genera, which are not distributed
in the North Atlantic, occur in the South Atlantic. Most of these species have a circumpolar
distribution in the southern hemisphere. Therefore, for the identification of these species,
it is advisable to use the keys and/or descriptions of the species in the Indian Ocean
(Nafpaktitis and Nafpaktitis, 1969; Nafpaktitis, 1978) and in the eastern Pacific (Wisner,
1976) in combination with those of Nafpaktitis et al. 3 (1977). It should be noted that con-
siderable improvement and changes in genus and species names are found in Nafpaktitis (1978).

Indian Ocean: For the identification of the species of Diaphus the description of 36
species by Nafpaktitis (1978) is the most up-to-date and comprehensive. For the fishes of
other genera, the description of 54 species by Nafpaktitis and Nafpaktitis (1969) and Kotthaus
(1972) are helpful, although their sampling coverage is restricted to the western Indian
Ocean. The species of the genera Protomyotophwri and Eleatrona show a circumpolar distribution
in the antarctic region and some of them do not occur in the sampling areas of the above two
works. But they are included in the keys for the eastern Pacific myctophids by Wisner (1976)

- 4 -

owing to their circumpolar distribution. Generally, faunal surveys have not been adequa
in the eastern and southern Indian Ocean and more taxonoraical study is needed in the get)
LampanyctuSj Syrribolophorus and Gyrnnoscopelus to establish the identification of the Indi
Ocean myctophid fishes.

Pacific Ocean: Wisner (1976) compiled available information on the taxonomy and di
tribution of 147 species of the eastern Pacific. His keys to these species are helpful
identifying not only the eastern Pacific species, but also the species of the central an
western Pacific and the Indian Ocean. He noted that the key is a preliminary one and it
complete in regard to the genera Diaphus, Gyrnnosoopelus and Lampanyctus . Bekker (1964,
1965) and Bekker and Borodulina (1976) studied the taxonomy and distribution of the sler
tailed myctophids, Loweina^ Tarlentonbeania^ Gonichthys, Centrobranchus, Hygophwn and Mz,
phwn in the entire Pacific and Indian Oceans. In the western Pacific, Kawaguchi and Aic
(1972) studied the taxonomy and prepared a key to the species of Myetophum. Kawaguchi t
Shiraizu (1978) did the same for Diaphus in the eastern Indian Ocean, the Southeast Asiar
Seas and the western Pacific.

Table 2.1

Families of fishes occurring in the mesopelagic zone
(based on Lindberg, 1971)

Families

!
Number of
genera

Families

Number oi
genera

Argentinidaea

Alepisauridae

Bathylagidae

Scopelarchidae

Opisthoproctidae

Evermanne 1 1 idae

Gonostomatidae

Giganturidae

Sternoptychidae

Nemichthyidae

ca. 5

Stomiatidae

Trachypteridae

Chauliodontidae

Regalecidae

Astronesthidae

Lophotidae

Melanostomiatidae

ca. 15

Melamphaeidae

Malacosteidae

Anoplogasteridae

Idiacanthidae

Chiasmodontidae

Myctophidae

ca. 30

Gempylidae

Paralepididae

Trichiuridae

Omosudidae

Centrolophidae

Anotopteridae

Te t r agonur idae

- 5 -

Generally, the taxonomy of the Pacific myctophids is not yet complete especially in
regard to the genera Lampanyotus, Gyrnnosoopelus and Diaphus, and more faunal survey is
needed in the central South Pacific and the Southeast Asian Seas. But studies on the myc-
tophid fishes of Australia, the Southeast Asian Seas, New Caledonia and the western tropical
Pacific have made steady progress and are nearing completion (pers. comm. B.C. Nafpaktitis
and J.R. Paxton) , These studies will certainly contribute to understanding of the taxonomy
and distribution of the myctophids in the entire Pacific and world oceans as well.

2.3 Identification of gonostomatid fishes

The taxonomy of gonostomatid fishes is more developed than that of myctophids. The
backbone of the taxonomy at the generic level was established by Grey (1960, 1964) and was
not significantly changed until Weitzman (1974) proposed a new evolutionary classification
(Table 2.2). For convenience, we follow Grey's system, since most previous works have
followed it. Twenty-one genera are included in her key to the genera of the world oceans,
although the genus Neophos was later synonymized with Torophos by Weitzman (1974). At
present, the total number of gonostomatid genera is 20 as shown in Table 2.2. Of these 20
genera, Yarrella, Polymetme and Argripnue have been collected by bottom trawl and are not
thought to be meso- or bathypelagic, while fishes of the other 17 genera are. Of the 17
genera, Cyclothone with 11 species and Gonostcma with 7 species are the largest, while
seven genera, Triplophos, Pollichthys, Photiohthys, Bonapartia, Margrethia, Denaphos and
Maourolicus, are monotypic. Diplophos and Vinciguerria include four species, lohthyoaoccus
and Valenciennellus three species and Woodsia, Thorophos, Avaiophos and Sonoda two each.

For identification at both the generic and species level, Grey's (1960, 1964) keys and
descriptions are still helpful, although her description is restricted to northwest Atlantic
species, and the keys are not prepared to deal with some taxonomically confused genera such
as Cyclothone and Diplophos. But most of this confusion has been recently resolved and
their distributions have been clarified by Berry and Perkins (1966), Mukhacheva (1967, 1972,
1974, 1976), Kawaguchi (1971) and Kobayashi (1973), who give comprehensive descriptions and
keys to species of a specific area or the world oceans. Several species new to science
were also described during the last decade, i.e., Avaiophos eastropas Ahlstrom and Moser,
1969; Diplophos greyae Johnson, 1970; Diplophos rebainsi Kreft and Parin, 1972; Woodsia
meyerwaardeni Kreft, 1973; Gonostoma longipinnis Mukhacheva, 1972; Cyclothone sumiae
Kabayashi, 1973. Recent advances in gonostomatid taxonomy are well reflected in the des-
cription of these species.

2.4 The new classification for stomiatoid fishes

Recently Weitzman (1974) proposed a new classification for stomiatoid families based
on their osteology. His classification resulted in considerable changes in the arrangement
of genera. He transferred seven genera from Gonostomatidae to Photichthyidae, which he
newly established, and seven genera from Gonostomatidae to Sternoptychidae, which had
formerly included only the three genera Polyipnus, Argyropeleeus and Stermoptyx as shown in
Table 2.2. In his classification, Sternoptychidae and Photichthydae are given high rank
together with Gonostomatidae and Myctophidae as families with abundant micronektonic fishes.
We compare his system to the older one in Table 2,2, since many workers may adopt his system
in the near future and some confusion may occur among workers unfamiliar with stomiatoid
taxonomy over the definition of the families Gonostomatidae, Sternoptychidae, and Photich-
thyidae .

Table 2.2

Comparison of the classification systems of
Grey (1960, 1964) and Weitzman (1974)

Grey (1960, 1964)

Weitzman (1974)

Gonostomatidae (20 genera)

1. Diplophos Gunther

2. Triplophos Brauer

3. Bonapartia Goode and Bean

4. Margrethia Jespersen and Tuning

5. Gonostoma Rafinesque

6. Cyclothone Goode and Bean

Gonostomatidae
(6 genera)

Thorophos Bruun*

AvaiophoQ Grey

Maurolicus Cocco

DanaphoG Bruun

Valenciennellus Jordan and Evermann

Argyripnus Gibert and Cramer

9.
10.
11.
12.
13. Sonoda Grey

Sternoptychidae

(10 genera including
Polyipnus Cocco
Argyropelecus Gunther
Sternoptyx Herman

14. Vineiguerria Jordan and Evermann

15. Polymetme McCulloch

16. Yarrella Goode and Bean

17. Polliehthys Grey

18. Photiohthys Grey

19. Woodsia Grey

20. Ichthyocoocus Bonaparte

Photichthyidae
(7 genera)

* Including Neophoe Myers

- 7 -

3. ECOLOGY AND LIFE HISTORY

3.1 Vertical distribution and migration

An extensive diel vertical migration is one of the most striking features of most meso-
pelagic fish species. During the day most fish are concentrated in one to several deep scat-
tering layers (DSL) found at depths from about 200 tn downwards (Fig. 3.1), but net sampling
and direct observations from bathyscaphes have both indicated that mesopelagic fish can also
be found between these layers (Bradbury et al.> 1971; Barham, 1971). Light seems to be a
major factor regulating the depth of the DSL (Kampa, 1971) (Fig. 3.2), and particular species
often seem to follow particular isolumes during migration (Boden and Kampa, 1967; Clarke,
1971). During nighttime some species (e.g., many myctophid genera) come to the surface
layer, while others stop at intermediate depths, or do not migrate at all (Pearcy and Laurs,
1966; Badcock and Merrett, 1976). Some authors (e.g., Gorelova, 1977) classify the surface
migrating forms as nictoepipelagic and those spending all the time within the mesopelagic
zone as mesopelagic . There also seems to be some ontogenetical variation in migration habits
(Nafpaktitis, 1968; Clarke 1973, 1974) and there are indications that some fish migrate to the
upper layers some nights while they stay at depth other nights (Clarke, 1971). Usually the
fish are most concentrated when in the DSL during the day (Taylor, 1968), but very dense con-
centrations have also been observed in the surface layer during the night (Gjrfsaeter, 1978a) .
There are also a few observations of myctophids coming to the surface during daytime (Alverson
1961), but this phenomenon seems to be of no general importance.

3.2 Seasonal variation

Data related to seasonal variation are few and partly contradictory. Haigh (1971) and
Ponomareva (1974) found only negligible seasonal variation during acoustical studies of DSLs,
while Donaldson and Pearcy (1972) found more pronounced variation. The reproduction of meso-
pelagic fish shows seasonal variation even in tropical and subtropical waters, and associated
with this, variation has also been observed in both size distribution and biomass of indiv-
idual species and of the total mesopelagic assemblage (Gibbs et al. 9 1971; Clarke, 1973).
In temperate waters seasonal variations have also been observed (Pearcy, 1977).

3.3 Horizontal distribution

The distributional patterns of mesopelagic fish generally seem to coincide with water
mass distribution (Cohen, 1973). Some species have a restricted area of distribution, while
others are found in all oceans. Most mesopelagic fish are oceanic, but neritic distributions
have also been observed (e.g., Clarke 1973).

The horizontal distribution of biomass depends on production at lower trophic levels
(Blackburn, 1977), and also on local factors. Off Oregon, Pearcy (1976) found the highest
biomass about 45 nautical miles offshore, while Gjrfsaeter and Blindheim (1978) observed the
highest biomass of mesopelagic fish a few miles off the 200 m depth contour off Northwest
Africa (Fig. 3.3) .

There are few indications of horizontal migration among mesopelagic fish, and special
feeding or spawning concentrations have not been reported. The larval distribution (e.g.,
Ahlstrom, 1972) seems to support the hypothesis of no spawning migration. It has, however,
been suggested that mesopelagic fish may be expatriated in areas with strong currents (O'Day
and Nafpaktitis, 1967; Zurbrigg and Scott, 1972; Gj^saeter, 1978). These expatriated
specimens are probably lost to the reproductive part of the population.

3.4 Behaviour

Aspects of the behaviour of mesopelagic fish have been observed by means of trawls,
acoustic instruments and submersibles. Some species may be found in schools, or aggregated
in scattering layers, or dispersed (Pearcy and Laurs, 1966; Backus et al. f 1968; Gj^saeter,
1978a) .

Figure 3.1 Echo recording showing vertical migration of

mesopelagic fish off West Africa, November 1972.
(From Gjisaeter and Blindheim , 1978)

- 9 -

IRRADIANCE (^W/cm 2 /nm)
I0 -5 IO" 4 10' 3 IO' 2 IO' 1 10 IO 1 10 2

600

Figure 3.2 Relationships between irradiance at
480 nm throughout the water column and
midday depth of a DSL in some regions
of the eastern north Atlantic and the
Gulf of California. Stippled areas
indicate vertical extent of DSLs.
(From Kampa, 1971)

- 10 -

3000 r

2000 .

1000

250

200 150 100

DISTANCE N MILES

- 1000

- 2000

3000

3000 r

>
o

cr
o

20CO

1000

250

200

150 100

DISTANCE N MILES

.1000

H-
UJ

.2000

3000

Figure 3.3 Integrated echo intensities refer to mesopelagic
fish showing concentrations of fish near the
shelf break off West Africa, November 1972.
(From Gjrfsaeter and Blindheim, 1978)

- 11 -

During daytime Barham (1971) often observed mesopelagic fish hanging motionless in the
water with head up or down, apparently in a state of torpor. During night the fish in the
upper layers were active, swimming horizontally, while fish staying at depth often were
observed to be immobile and vertically oriented. The torpid fish were, however, capable of
rapid evasive movements when approached by the submersible.

The ability to avoid nets depends on behaviour and on the efficiency of the sensory
organs. Both surface and underwater observations show that the myctophids are capable of
rapid swimming when attacked by predators, but they seem soon to become exhausted. The
possibility of avoiding a net is dependent on orientation relative to the path of the net
(Harrison, 1967; Scully-Power, 1977). Therefore the vertical position often observed during
daytime may partly explain the better catches made during the night than during the day
(Pearcy and Laurs, 1966). Barham (1971) also suggested that fish actively swimming to feed
or to avoid predators may be nearer the exhaustion point than those resting inactively, and
therefore less able to avoid the net.

3.5 Anatomy and physiology

Except for features directly linked to classification, and the swimbladder, anatomical
and physiological studies on mesopelagic fish are few (Jollie, 1954; Paxton, 1972; Weitzman,
1974) . The swimbladder has been more intensively studied as it is of primary importance to
the sound reflecting properties of these fishes. Many species, especially among the deepest-
living forms, have no swimbladder (Marshall, 1960). Other species have a gas-filled swim-
bladder when young which becomes filled with fat in older age, and it has even been observed
that among fish apparently of the same species, age and sex, and caught in the same area at
the same time of the year, individuals with either fat-filled or gas-filled swimbladders may
be found (Butler and Pearcy, 1972).

Most species, at least among those inhabiting the upper part of the mesopelagic zone,
have gas-filled swimbladders. The size of these bladders determines the acoustic properties
of the fish. Brooks (1977) recently studied the size of the swimbladder in 55 mesopelagic
fish species. He observed large intra- and interspecific variation, but in general the
bladder size was smaller than the 5 percent of body volume commonly assumed. His values
were also considerably smaller than those observed by Shearer (1971). Differences in methods
may be partly responsible for this discrepancy.

Various aspects of the physiology of mesopelagic organisms were recently reviewed by
Childress (1977). Fat content and fat composition have been studied in a few mesopelagic
fish species (Table 3.1). The results indicate extensive variation among species. A high
content of wax esters seems to be common (Nevenzel et al., 1969), and this may be an obstacle
to utilizing these fish for human consumption (Kinumaki et al. 9 1977).

3.6 Position and importance in the food web

Mesopelagic fish constitute a major part of the biomass in oceanic areas (Ahlstrom,
1969; Clarke, 1973). Yet their position in the food web is poorly understood. Data on the
food of mesopelagic fish have accumulated and were recently reviewed by Hopkins and Baird
(1977). Crustacea appear to be the principal forage (Table 3.2) with copepods, euphausiids,
ostracods, amphipods and small decapods being the most important items. In addition to
species variation, ontogenetic, seasonal and regional variation is observed (Gj^saeter,1973b;
Hopkins and Baird, 1973; Gorlova, 1974; Tyler and Pearcy, 1975), Generally it seems that
most mesopelagic fish are opportunistic feeders, consuming any available food falling within
the size limits which they can manage. Exceptions are, however, observed (Table 3.2). The
prey taken is partly herbivorous and partly carnivorous. Some studies indicate that herbi-
vorous plankton dominate (Legand and Rivaton, 1969; Baird et al., 1975), but the data avail-
able are too sparse to assess the relative importance of these components for most mesopelagic
species.

- 12 -

Table 3.1

Lipid content of some mesopelagic fishes
with percentage of wax content in total lipids

Species

Lipid
in fresh body wt.
(%)

Wax ester
in total lipid
(%)

Author

Gonostomatidae

Cyolothone atvaria

4.0

58.4

Kayama and Ikeda, 1975

C. pseudopa 1 lida

A. 3

53.5

Kayama and Ikeda, 1975

C. alba

5.3

54.0

Kayama and Ikeda, 1975

C. palHda

2.3

34.0

Kayama and Ikeda, 1975

Gonostoma graoile (?)

2.5

20.0

Kayama and Ikeda, 1975

G. gracile (t)

2.9

15.9

Kayama and Ikeda, 1975

Maupolicus muelleri

5.5

15.3

Kayama and Ikeda, 1975

M. muelleri (silage)

3.1

Anon, 1977a

Myctophidae

Benthos ema pterotum

4.6

B. Myrseth (in prep.)

Hygophwn reinhardti

3.3

Nevenzel et al. y 1969

Myotophum nitidulum

3.8

10.6

Kayama and Ikeda, 1975

M. asperwn*

2.0

17.2

Kayama and Ikeda, 1975

Symblophorus evermanni

3.1

Nevenzel et al.> 1969

S. calif orniensis

4.3

trace

Nevenzel et al., 1969

Tarletonbeania crenularis

2.1

5 (max . )

Nevenzel #t al.> 1969

Diaphus theta

15.8

trace

Nevenzel et al., 1969

D. coeruleus

10.2-

17.2

Melnikova, 1973

Z?. glandulifer
(= D. suborbitalis )

7.9

3.7

Kayama and Ikeda, 1975

D. fulgens*
(= D. kuroshio)

7.1

4.4

Kayama and Ikeda, 1975

D. latus

(= D. garmanni)

8.7

1.7

Kayama and Ikeda, 1975

Stenobrachius nannochir*

11.4

67.1

S. leuaopsarus

15.6

90.9

Nevenzel et al. 3 1969

Triphoturus mexicanus

14.5

82.2

Nevenzel et al., 1969

Lampanyctus rittevi

14.2

Nevenzel et al. f 1969

L. macroptems

11.9

3.7

Kayama and Ikeda, 1975

Lampanyctodes hectoris

no data

Centurier-Harris, 1974

Sternoptychidae

Sternoptyx diaphana*

1.4

4.2

Kayama and Ikeda, 1975

Chauliodontidae

Chauliodue sloani*

2.4

7.2

Kayama and Ikeda, 1975

* Excluding head. In other species not asterisked, whole fish was examined.

- 13 -

Table 3.2
Feeding behaviour of some mesopelagic fishes

Species

Area

Time of
Feeding

Selectivity

Author

Myctophwn spinosum

W. Equatorial
Pacific

night

Gorelova, 1974

M. aurolatewatum

night

Gorelova, 1974

Sympolophorus evevmanni

night

Gorelova, 1974

Benthosema glaciate

Norwegian fjords

night

random

Gjdsaeter, 1973

Benthosema glaciate

N.W. Africa

night

Kinzer, 1977

B. pterotwn

Arabian Sea

evening/
night

? random

Gj^saeter, 1978a

B. fibula-bum

evening/
night

Gj^saeter, 1978a

Diaphus taaningi

Off Venezuela

night

Baird et al., 1975

D. theta

Off Oregon coast

night/

Tyler and Pearcy, 1975

morning

D. dinner Hi

N.W. Africa

acyclic

Samyshev and Schetinkin,
1971

Lobianchia dofleini

N.E. Atlantic

night

random

Merrett and Roe, 1974

Notolychnus valdiviae

N.E. Atlantic

night

?selective
copepods

Merrett and Roe, 1974

Stenobraehius leucopsarus

Off Oregon coast

night/

Tyler and Pearcy, 1975

morning

Tarletonbeania crenularis

Off Oregon coast

night/

Tyler and Pearcy, 1975

morning

Lanrpanyctus cupriarius

N.E. Atlantic

night

selective
amphipods

Merrett and Roe, 1974

Lepidophanes guentheri

N.W. Africa

acyclic

Samyshev and Schetinkin,
1971

Cyclothone acclinidens

Off California

night

De Witt and Calliet, 1972

Cyclothone signata

Off California

acyclic

De Witt and Calliet, 1972

Valenciennellus tri-
punctulatus

N.E. Atlantic

night

selective
copepods

Merrett and Roe, 1974

Maurolicus muelleri

N.W. Africa

acyclic

Samyshev and Schetinkin,
1971

Argyropelecus aculeatus

N.E, Atlantic

dusk

selective
ostracods

Merrett and Roe, 1974

A. hemigyrmus

N.E. Atlantic

dusk

random

Merrett and Roe, 1974

Leuroglossus stilibius

Off California

night

De Witt and Calliet, 1972

- 14 -

The time of feeding by some species is shown in Table 3*2. Diurnal feeders, nocturnal
feeders and acyclic feeders have been found.

Mesopelagic fish seem to partition food among themselves by species and ontogenetic
variation in depth distribution and feeding time (Legand et al. 9 1972; Kawaguchi, 1973;
Merrett and Roe, 197A; Clarke, 1973, 1974). Differences in breeding season (Gibbs et al. 9
1971; Goodyear et al. 3 1972) may also lead to a partitioning of food between the younger
stages.

It is very difficult to assess the daily ration of food consumed as the digestion rates
are not known. If it is assumed that the stomach is filled once every night in cyclic feed-
ers, data on stomach contents might be used to get an idea of food consumption. Baird et al.j
(1975) found a maximum gut content of about 0.8% of the body weight of Diaphus taaningi, and
Legand and Rivaton (1969) found similar percentages for various tropical myctophids.

There are many observations of various species of fish and other marine animals feeding
on mesopelagic fish (see Table 3.3), but the importance of mesopelagic fishes in the diet
of these animals is not clear.

Most commercial fish species feed above the shelf where mesopelagic fish species are
not usually found. When they meet at the shelf edge there are examples showing that meso-
pelagic fish may be an important food item (e.g. Pereyra et al.* 1969).

There is some conflicting evidence about tuna feeding on mesopelagic fish. Legand et
at., (1972) and Roger and Grandperrin (1976) conclude that mesopelagic fish is of no import-
ance as food for tuna. Others (see Table 3.3) have found that they may be a significant
component in the diet of tuna species. It has also been shown that swordfish may eat meso-
pelagic fish although volumetrically they are of small importance (Scott and Tibbo, 1968).

Borodulina (1972) has shown that Chauliodontidae, Stomiatidae and various other deep-
water predatory fish feed on Myctophidae and Gonostomatidae, while Fourmanoir (1969) con-
cluded that Alepisaurus usually does not.

Several authors (e.g. Fitch and Brownell, 1968; Mead and Taylor, 1953) have shown that
marine mammals may feed on mesopelagic fish, and for some species they may be an important
part of the diet.

Cephalopods are another group of mesopelagic fish predators (e.g. Zuev and Nesis, 1971)
and in some areas they may be the most important group.

3- 7 Growth, mortality and production

Length distributions have been reported for a number of mesopelagic fish (e.g. T&ning,
1918; Gibbs et al. 9 1971; Krueger and Bond, 1972; Goodyear et al., 1972; Clarke, 1973,1974),
but although a size distribution can sometimes be interpreted in terms of age, age is usually
unknown, so the growth schedule cannot be determined. For mesopelagic fish inhabiting cold
or temperate waters age can be read from the otoliths and growth can therefore be calculated.
The growth of some of these species is shown in Table 3.4. The range in the growth co-
efficient, K, from the von Bertalanffy equation

l t L > < ' - ex P [ - K ^V J >

for the non-tropical species is 0.11 - 1.05 and the theoretical maximum lengths, L^ range
between 49 and 119 mm. The data for Benthosema glaciate show that the growth parameters
may vary extensively even within the same species and in the same general area. Typical
growth curves are shown in Fig. 3.4.

- 15 -

Table 3.3
Some studies of animals feeding on mesopelagic fish

Predator

Area

Author

Oneorhyneus sp.
Oncorhyncus sp.
Gadus morhua
Gadus morhua
Sebastes rnarinus
Sebastodes flavidus
Sebastodes sp.
Trachurus symonetricus
Thunnus alalunga
Thunnus alalunga.
Thunnus albacares
KatsuDonus pelamis
Thunnus thynnus
Xiphias gladius
Scomber japonicus
Glossanodon senrifasciatus
Theragra chalcogramma
Arotoscopus japonicus
Todarodes paoifious
Stenella caeruleoalba
Various cetacea
Callorhinus ursinus

Off Washington

N.E. Pacific

Newfoundland

Off Oregon

Off California

E. Tropical Pacific

Off California

N.W. Atlantic

Japan Sea

Off Central Japan

Off Northern Japan

Shimada, 1948
Manzer, 1968
Popova, 1963
Kashintsev, 1963
Kashintsev, 1963
Pereyra et al., 1969
Fitch, 1951
Fitch, 1951
McHugh, 1952
Pinkas et al., 1971
Alverson, 1963
Alverson, 1963
Pinkas et al. 3 1971
Scott and Tibbo, 1968
Nishimura, 1959
Okiyama, 1971
Nishimura, 1960
Okiyama, 1971
Okiyama, 1965
Miyazaki et aZ.,1973
Fitch and Brownell, 1968
Jap. Fish. Agency, 1965

For tropical species, age determination is still tentative. Rings on the otoliths,
supposed to be formed daily, have been used to age various shallow-water species (Panella,
1974; Brothers et al., 1976; Taubert and Coble, 1977). Gjrfsaeter (1978, 1978a) and Gjrfsaeter
and Blindheim (1978) used the same procedure to age mesopelagic fish species, but the valid-
ity of this method has not been confirmed. Growth curves based on this tentative aging
technique shows K between 1.31 and 5.62 and L M from 68 to 77 mm (Fig. 3.5).

Generally, it seems that mesopelagic fish from cold waters are slow growing although growth
may be rapid during the first part of life. Warm-water mesopelagic fish seem, however, to
have a fast growth, and most of them probably reach maximum size in one year or less. Some
species (e.g. MauroliQud muelleri* Diaphus subovbitale) seem to have a fast growth until
sexual maturity is reached (Gj^saeter, 1978; Go et al*> 1977) and a very slow growth later.

- 16 -

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4 5
AGL- YEARS

Figure 3.4 Growth of Benthosema glaciate from western Norway.
The points represent mean length of an age group
within a sample and the vertical bars 95% confidence
limits. (From Gjrfsaeter, 1978)

- 18 -

80-i

70-

50-

40-

3O-

000

20-

10-

50 IOO ISO 2OO 25O 3OO 35O 4OO

NO OF ZONES

Fig. 3.5 Relationship between length and number of primary
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- 19 -

Seasonal variation in growth rate has been observed in Benthosema glaciate (Gjdsaeter,
1973) and Diaphus suborbitalis (Go et al., 1977), the former growing faster in winter and
the latter in summer. The differences in growth rate may be related to hydrographic factors,

Most tropical myctophids and smaller gonostomatids seem to have a one-year life cycle
(e.g. Clark, 1973, 1974). Based on the tentative ageing carried out by Gj^saeter and
Blindheim (1978) and Gjrfsaeter (1978a), Benthosema pterotum* B. fibulatum and Diaphus dumer-
ili all have instantaneous mortality rates of about 5, corresponding to an annual mortality
of more than 99%.

Mesopelagic fish from colder waters have a longer life cycle, but few estimates of
mortality rates have been made. Gj4saeter (1973, 1978) estimated the mortality of Bentho-
sema glaciate, Notoscopelus kroeyeri and Maurolicus muelleri to be about 0.7, 0.8 and 1.8
respectively, but due to selectivity of the gears used and other sources of error, these
results are only tentative.

Estimates of annual production are also few, and may be strongly biased. For Notosco-
pelus kroeyeri, Gj^saeter (1978) got 5.9 g/recruit (recruited at one year old). For Ben-
thosema glaciate and Maurolicus muelleri, both recruited at time t Q , the corresponding
values are about 0.4 and 0.2 g respectively (Gjdsaeter, 1978), For tropical species the
production could be higher than the standing stock.

3.8 Fecundity

Knowledge of the fecundity of mesopelagic fishes is quite limited. The data accumulated
so far for fishes of the Myctophidae and Gonostomatidae are listed in Table 3.5. Among the
myctophid fishes, the relation between log F and log L (F fecundity; L * body length) is
similar both intra- and interspecifically. Gj^saeter (1978) reported that in Benthosema
glaciale, log F is related to log L by the following equation, suggesting that F is approx-
imately proportional to the body weight:

log F = 3.44 log L - 3.21 (r 2 = 0.866)

In Fig. 3.6, the fecundity of different species of myctophid fishes is plotted against body
length. There is a roughly linear relationship between log F and log L, expressed by the
following equation;

log F = 3.16 log L - 5.11

Fig. 3.6 shows that the equation is a good fit. At the present stage of study, this empir-
ically derived relationship may be useful in estimating the fecundity of other myctophid
species not yet studied, but we do not have enough data to be sure the relationship holds
generally.

Among the fishes of the Gonostomatidae, no relationship between F and L such as that
observed in the Myctophidae has yet been found, although the data accumulated so far are
very few. In three species, Cyclothone braueri* C. microdon and Maupolicus muelleri* fecund-
ity has no relationship to body length (Badcock and Merrett, 1976; Okiyama, 1971; Gj^saeter
1978), but in Valenciennellus tvipunctulatus, the number of eggs increases with animal size
(Badcock and Merrett, 1976).

Latitudinal variation in fecundity is reported in Cyclothone braueri and C. microdon>
with higher fecundity in a productive area above 30N latitude in the northeast Atlantic
(Badcock and Merrett, 1976).

- 20 -

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- 22 -

3.9 Spawning seasons

Spawning seasons for myctophid and gonostomatid fishes have been deduced from the
maturity stage of the ovary and the seasons in which the larvae occur. The data accumulated
so far are listed in Tables 3.6 and 3.7 with their sources. Studies in tropical regions
are still few.

Myctophid fishes in temperate and subtropical regions spawn mainly from late winter to
summer. Since zooplankton production lags behind the peaks in primary production only
slightly, the period of principal spawning for most myctophid species appears to be timed
to coincide with the seasonal peak in the production of zooplankton, the main food source
(Clarke, 1973). In the subarctic region, Stenobraehius l&ucopsarus is known to spawn in
winter (Smoker and Pearcy, 1970). The subantarctic species Lampany ctodea hectorie also
spawns in winter off New Zealand (Robertson, 1977). Winter spawning in high latitudes
seems to be an adaptation to low water temperature, since hatching takes much longer there
than in low latitudes. Even in the middle latitudes, the deeper-living species, Taaning-
iehthys minimum, Lampanyctus niger and L. nobilis, tend to have an earlier or longer spawn-
ing season. This also might be due to a lower water temperature.

The spawning season of gonostomatid fishes shows a greater variation than that of
myctophids, probably reflecting the differences found in their manner of vertical migration,
feeding habits, and breeding habits (e.g. sexual dimorphism and sex reversal). For example,
the spawning season for species of the genus Gonostoma, which are not active diel vertical
migrants, differs among species, G. elongatum spawning in spring and summer, G. ebelingi
in early fall, G. atlanticum during all seasons in the subtropical central Pacific off
Hawaii, and G. gracile in fall and winter in the western Pacific (Clarke, 1974; Kawaguchi
and Marumo, 1967). Their spawning does not seem to be timed in accordance with the seasonal
peak in the abundance of their food. Among species of the genus Cyclothonc, all of which
are non-migrants, a shallow- living species, C. bivuerij is known to spawn in spring and
summer, but the deeper-living species C. pseudopallida* C. pallida 3 and C. atraria spawn
several times a year or during all seasons, or so it would appear from the obscurity of the
modes in their size frequency distribution (Badcock and Merrett, 1976; Kawaguchi, unpub-
lished data). On the other hand, the species Maurolicus muelleri> Vineeguerria nimbaria
and V. .poweriae, which migrate up to the epipelagic zone at night, spawn mainly in spring
and summer as observed in many myctophid species (Nishimura, 1959; Clarke, 1974; Gjiisaeter,
1978).

Generally, mesopelagic species in higher latitudes and shallower habitats that are
active diel vertical migrants, tend to have a better-defined spawning season than species
in lower latitudes and deeper habitats or species of non-migrants . In other words, the
migrating or shallow mesopelagic species can sharply perceive and react to the seasonal
change in the environmental factors, resulting in a more distinct spawning season.

In regard to the number of spawnings per year, Tuning (1918) could not confirm multiple
spawning during his extensive study on the Mediterranean myctophid fishes, but it has been
reported for the gonostomatid species Gonoetoma ebelingi and Cyclothone pseudopallida
(Clarke, 1974; Badcock and Merrett, 1976). However, at present it is difficult to distin-
guish multiple spawning from ordinary spawning, since the spawning season is usually long
and sometimes ill-defined for many mesopelagic species.

3.10 Life span and age at maturity

Our knowledge of the life span and age of maturity of mesopelagic fishes is confined
to a few species of the Myctophidae and Gonostomatidae, In subtropical and tropical waters,
the small myctophid fishes Notolyohnus valdiviae, Benthoeema suborbitale, Diaphus schmidti,
D. andereeni and Lobianchia dofleini are reported to attain sexual maturity about one year
after hatching at a size of 25 to 40 mm SL, and the medium-sized myctophids Benthosema
pterotwn, B. fibulatwn, L. dumerili and D. euborbitalie at 50 to 70 mm SL. After breeding,
their abundance is known to decrease abruptly, probably owing to death after spawning

- 23 -

Table 3,6

Spawning season of the species of the family Myctophidae
(Sp. spring; S urn. summer; F.=fall; W.=winter)

Species

Main spawning
season

Areas

Sources

Benthoeema glaciate
B. glaoiale
B. glaciate
B. suborbitale
Bolinichthys longipes

B. supralateralia
Ceratoscopelus maderensie

C. wamingi
Diaphus anderseni

D. brachycephalus
D. elucence

(= Z>. perspicillatus )
D. fragilis
D. holti
D. rafinesquei
D. rolfbolini

(= D. phillipsi)
D. schmidti
D. suborbitalis
Electrona rissoi
Hygophwn hygomi
H. proximum
H. reirihardti
Lampanyctodes hectoris
Lampanyctus alatus

L. niger
L. nobilia
L. ateinbecki
Lampadena Iwninosa
LobiancHia dofleini
L. dofleini
L. gemellarii
L. gemellarii
Myctophwn affine

(= M. nitidulum)
M. benoiti

(=Hygophum benoiti)
M. httmboldti (=Syrribolo-

pholus veranyi)
M. punctatum

M. selenoidea

(= M. eelenops)
Notolychnus valdiviae

Notoecopelue elongatus
Stenobrachiue leucopsarus
S. leucopsc&ue
Taaningichthya minimus
nigrescens

W. to Sp.

Mediterranean

TSning, 1918

Early Sp.

W.N.Atl.(40-50N)

Halliday, 1970

Sum*

Off western Norway

Gj^saeter, 1978

Sp.

Off Hawaii Clarke, 1973

Sp. to Sum.

Off Hawaii Clarke, 1973

Sum. to F.

Off Hawaii Clarke, 1973

Sum.

Mediterranean Tuning, 1918

Sp. to Sum.

Off Hawaii Clarke, 1973

Sp. to Sum.

Off Hawaii Clarke, 1973

Sp. to Sum.

Off Hawaii Clarke, 1973

Sum.

Off Hawaii Clarke, 1973

Sum.

Off Hawaii Clarke, 1973

Sum.

Mediterranean

TSning, 1918

F. to W.

Med i terranean

Tuning, 1918

Sp. to Sum*

Off Hawaii

Clarke, 1973

Sp. to Sum.

Off Hawaii Clarke, 1973

Sp. to Sum.

Suruga Bay, Japan Go et al. 9 1977b

Sp. Sum. F.W. (?)

Mediterranean Tuning, 1918

F. to W.

Mediterranean TSning, 1918

Sp. to early Sum.

Off Hawaii Clarke, 1973

Sp. Sum. F.W. (?)

Off Hawaii

Clarke, 1973

W. to Sp.

Off New Zealand

Robertson, 1977

Sp. Sum. F.W. with

Mediterranean

Tining, 1918

a peak in Sp . to

Sum.

Late Sum. to W.

Off Hawaii

Clarke, 1973

Sp. Sum. F.W.(?)

Off Hawaii

Clarke, 1973

Sp. to W.

Off Hawaii

Clarke, 1973

Sp. to Sum.

Off Hawaii Clarke, 1973

W. to Sum.

Med i terranean

lining, 1918

W, to Sp.

w.N.Atl.(32N,64W)

Karnella & Gibbs, 1977

Sp. to Sum.

Off Hawaii

Clarke, 1973

Mediterranean

Tuning, 1918

Sp. to Sum.

Off Japan (35-40ON)

Odate & Ogawa, 1961

Sp. to F.

Mediterranean Tuning, 1918

Sp. to F.

Mediterranean

lining, 1918

W. to Sum. with a

Mediterranean

lining, 1918

peak in Sp.

Sp.

Off Hawaii

Clarke, 1973

Sp. to Sum. with a

Off Hawaii

Clarke, 1973

peak in Sum.

W. to Sum.

Mediterranean lining, 1918

Off Oregon, USA Smoker & Pearcy, 1970

W. to Sp.

Off California, USA Fast, 1960

W. (?)

Off Hawaii

Clarke, 1973

Sp. to Sum.

Off Hawaii

Clarke, 1973

- 24 -

Table 3.7

Spawning seasons of some species of the family Gonostomatidae
(Sp.spring; Sum.*summer; F.=fall; W.-winter)

Species

Cyolothone braueri
C. braueri

C, braueri

C. pseudopallida

Danaphos ooulatus
Gonostoma atlantiawn
G. ebelingi
G. elongatwn
G. elongatwn
G. graeile

Maurolicus muelleri
M. muelleri
M. muelleri
M. muelleri

M. muelleri
M. muelleri

Va lenoienne 1 lus
tripunctulatus

Vinoiguerria nimbaria
V. poweriae

Main spawning
season

Sp. to Sum.
Sp. to Sum.

Sum.

Spawning more than
once a year (?)

Sp. Sum. F. W. (?)

Sp. Sum. F, W.

Early Sp. & early F.

Sp. to Sum.

F, to early Sp.

;Sp. to F

's p .

W. to Sp.

Sp. to F. with a
peak in Sum.

Sp. to Sum.
Sp. to Sum.
Sp. Sum. F. W.

Sum. to F.
Sp. to Sum.

Areas

e.N.Atl.(30N,23W)
Mediterranean

w. Mediterranean
e.N.Atl.(30N,230W)

Off Hawaii
Off Hawaii
Off Hawaii
Off Hawaii
Subtropical Atl.
Off Japan

Off w. Norway
Japan Sea
Off Morocco
e.N.Atl.(59N,190W)

Off Norway
Off Norway
Off Hawaii

Off Hawaii
Off Hawaii

Sources

Badcock and Merret,
1976

Jespersen & Taning,
1926

Goodyear et a 1. 9 1972

Badcock & Merrett,
1976

Clarke, 1974
Clarke, 1974
Clarke, 1974
Clarke, 1974
Krueger & Bond, 1972

Kawaguchi & Marumo,
1967

Wiborg, 1954
Nishimura, 1959
Grey, 1964
William & Hart, 1974

Gjtfsaeter, 1978
Bjorke, unpublished
Clarke, 1974

Clarke, 1974
Clarke, 1974

- 25 -

(Clarke, 1973; Karnella and Gibbs, 1977; Gjfisaeter, 1978). The life span of many other
medium-sized species in the subtropical and tropical waters remains to be determined. On
the basis of growth rates, they probably become sexually mature one year after hatching in
highly productive areas, but in areas of low productivity it may take more than two years.
The larger myctophid fishes Lampadena luminosa, Bolinichthys supra lot era Ms, and Notoscope-
lus aaudispinosus appear to need more than two years to reach their size maturity, 70-100 mm
SL (Clarke, 1973). In temperate or subarctic waters, not only the larger species Steno-
brachius leucopsartLS > but also the medium-sized species Myctophwn affine (= M. nitidulwn)
and Benthosema glaciate, need two to four years after hatching to attain maturity and their
life spans are usually reported to be three to five years (Odate, 1966; Halliday, 1970;
Smoker and Peaarcy, 1970; Gj^saeter, 1978).

In the family Gonostomatidae, the smaller species Valenciennellus tripunetulatus and
Vinaiguerria nimbaria, are reported to mature within one year after hatching, but the larger
fish Gonostoma elongatum needs more than two years (Clarke, 1974). Studies on the life span
of gonostomatid fishes are very few. Gj^saeter (1978) recognized three age groups in Mauro-
licus muelleri in Norwegian waters based on an otolith analysis. In the western north
Pacific, Gonostoma gracile attains sexual maturity as a malt one year after hatching, then
reverses its sex to female and spawns again at the end of its second year. But cohort
size abruptly decreases in the third year, probably due to death just after the second
spawning.

3 11 Dwarf males, sex reversal and hermaphroditism

Dwarf males and sex reversal have, been known to occur among meso- and bathypelagic
species. Marshall (1971) pointed out the ecological significance of these phenomena as an
adaptation to a food-poor environment. His hypothesis is supported by the fact that these
phenomena are restricted to genera such as Gonostoma and Cyolothone that do not make a diel
vertical, migration to feed in the food-rich epipelagic zone (Kawaguchi and Marumo, 1967;
Marshall, 1971; Clarke, 1974; Badcock and Merrett, 1976). We cannot find such phenomena in
the vertically migrating myctophid and gonostomatid species, which spend the night feeding
in the productive epipelagic zone.

Dif f erent from dwarf males and sex reversal, hermaphroditism is considered to be an
adaptation related to breeding rather than feeding (Marshall, 1971). This phenomenon has
been widely reported in the families Paralepididae, Alepisauridae, Evermannelidae, Scope-
larchidae and Omosudidae (Mead, 1960; Gibbs, 1960; Mead, Bertelsen and Cohen, 1964; Merrett,
Badcock and Herring, 1973).

- 27 -

4. METHODS OF ABUNDANCE ESTIMATION
4.1 Net sampling

Catches made in various types of trawls are the most common source of information on
tnesopelagic fish, including abundance. Many types of trawls ranging from small micronekton
nets to large commercial trawls have been used, but Isaacs-Kidd midwater trawls (IKMT) and
similar gears with mouth areas of 1-10 m 2 have been most common.

When trawls are used there are two main sources of bias to be considered: avoidance
(animals sensing the approach of the net and swimming out of its path) and escapement
(animals pass through the meshes after entering the net). Both are dependent on size, be-
haviour, sensing and swimming ability of the fish, and escapement also on body form. Further,
they are dependent on the size of the gear, its mesh size, towing speed, etc.

In addition to the bias introduced by avoidance and escapement, the distribution of
sampling in relation to the distribution of fish is an important source of variance. Hor-
izontally, the sampling is usually randomly distributed, or distributed according to a pre-
determined grid of stations, and the fish abundance can, therefore be calculated directly
from the mean catch. If, however, the sampling is distributed according to acoustic record-
ings or other available information on fish distribution, a stratification of the data is
necessary to get an unbiased estimate.

Midwater trawl hauls are often oblique and the catch is supposed to represent the mean
density in the layer sampled. If the hauls are horizontal, several samples are needed to
assess the vertical distribution, or this distribution must be known from other sources.

Micronekton nets

Each kind of net has its own history of modifications to which many workers have con-
tributed. Here, however, we do not intend to review this history. The micronekton nets
which have been widely and effectively adopted are the Isaacs-Kidd midwater trawl (IKMT)
(Isaacs and Kidd, 1953), the rectangular midwater trawl (RMT) (Clarke, 1969; Baker et al.>
1973) and ring trawls, usually 113 or 160 cm in diameter.

All the nets hitherto developed have both merits and demerits related to their struc-
tures. IKMT has been widely adopted by many scientists engaged in mesopelagic faunal sur-
veys throughout the world because of its simple structure and ease of handling. It has the
advantage over ring trawls of not having large obstacles such as bridles. just in front of
the mouth, and having an effective depressor which allows high speed towing at a fixed
depth. For sampling at fixed depths with IKMT, catch dividing buckets or devices have been
developed (Pearcy and Habbard, 1964; Foxton, 1963; Brent, unpubl.), but due to technical
difficulties, all the devices are attached to the posterior part of the net instead of the
mouth part. These types of codend closing devices are prone to contamination which can be
overcome only by using a net equipped with an opening-closing device at the mouth, Isaacs
and Brown (1966) described a method for attaching an opening-closing Isaacs-Kidd trawl de-
vice at the mouth, but contamination problems still remain (Clarke, 1969).

Rectangular nets have a great advantage over many other types of nets in that the
mouth is unobstructed (Tucker, 1951; Davies and Barham, 1969; Clarke, 1969). An opening-
closing system at the mouth of the net has been developed for the rectangular net (RMT) by
scientists in the U.K. (Clarke, 1969; Baker et al., 1973).

Recently an RMT sampling system with the N.I.O. combination net (RMTi4.fi) has been
'developed to collect micronektonic animals over various size ranges (Baker et al.> 1973).
The system includes an acoustic telemetering system which monitors net depth, relative ve-
locity, total distance travelled, and water temperature, and acoustic opening and closing
devices. One problem with the RMT is that the precise volume of water filtered by the nets

- 28 -

cannot be estimated, since the angle of the mouth to the towing direction is variable,
depending upon the speed of the net through the water. But Baker et al. 3 (1973) noted that
for tows of equal duration "the inverse relationship between net area and distance travelled
combined with the standardization of the fishing procedure and the accurate monitoring of
the speed, will tend to minimize the difference between hauls to a level which is probably
of little significance compared to other errors inherent in this and any other comparable
sampling method (most notably, the irregular distribution of organisms)" and they felt that
the "RMT system has now reached a stage where the various nets provide a useful series for
reliable routine sampling of a wide size range of organisms in a consistent manner."

Many kinds of ring nets, large conical plankton nets, such as the Indian Ocean Standard
Net (Currie, 1962), ORI-net (Otnori, 1965; Omori et al., 1965), N M3 -net (Foxton, 1969) and
that adopted by Soviet scientists, etc., have been used to collect micronektonic animals.
The common demerit of ring nets is that the bridles probably scare off some active swimmers
(Currie, 1962). But its rigid metal ring and bridles assure precise measurement of the
volume of water filtered, which recommends ring nets for the sampling of small, inactive
swimmers, such as Cyelothone and larvae, postlarvae and juveniles of many species.

Sampling limitations of micronekton nets: Because of their small mouth opening, micro-
nekton nets are inadequate for catching larger fishes (more than ca. 5 cm), but they can be
caught in commercially-sized trawls (Harrison, 1967). Day-night differences in IKMT catches
were studied off southern California to determine biomass, numbers and mean sizes of fishes
and decapod crustaceans, resulting in the observation that daytime net avoidance is not a
problem with an oblique tow (Atsatt and Seapty, 1974). But the size of the animals studied
was usually less than ca. 5 cm. The influence of net speed on the IKMT catch depends not
only on species, but also on the size of the animals (Aron and Collard, 1969).

Commercial trawls

Commercial trawls have a large mouth opening (100 - 1 000 m 2 ), and usually very large
meshes (e.g. 20 cm) in the front part, gradually decreasing towards the codend. Due to the
large mouth, avoidance is probably negligible. Escapement through the meshes is, however,
a serious problem. Usually only the codend has a mesh small enough to retain most size
groups of mesopelagic fish. Commercial fish species are led by the large meshes in the
front part of the trawl and are finally caught by the finer meshes in the hind part. It is
not known, however, to what extent this applies to the smaller raesopelagic fish species.
Therefore, it is very difficult to assess the effective mouth opening.

Anon (1976) made two hauls with a 1 360 mesh pelagic trawl and a much smaller trawl,
fine-meshed throughout, designed to catch krill. The larger trawl had a mouth opening of
about 600 m 2 and the fine-meshed krill trawl about 300 m 2 . The codend mesh size was
approximately the same in the two nets. The experiments were carried out in an area with
very dense concentrations of Benthosema pterotwn. Although the comparisons were too few
to draw final conclusions, they suggest the two trawls catch equal quantities of myctophids,
This implies that many of the fish entering the mouth of the larger trawl escaped through
the larger meshes toward the front.

Based on these experiments it is supposed that the "effective" mouth area of the 1 360
mesh pelagic trawl is not larger than that of the krill trawl, i.e. about 300 m 2 . Later in
this paper a 1 600 mesh pelagic trawl which has a larger mouth area, but also larger mesh
size will be arbitrarily supposed to have the same effective mouth area.

Experiments carried out by Myrseth (in prep.) in the same area gave slightly different
results. He placed bags on various parts of the trawl to catch fish filtered through the
meshes. Out of ten experiments, only one gave a large catch of fish in the bags. Salps
were, however, often taken in the bags in quantities comparable to those found in the codend
From these experiments it is tentatively concluded that mesopelagic fish are led to some
extent by the large meshes in the trawl toward the codend.

- 29 -

Although it is very difficult to draw conclusions on the quantitative characteristics
of commercial-sized trawls, there is no doubt that they catch larger raesopalagic fish far
better than do micronekton nets, while small species such as Cyolothone may be underestimated
(Harrison, 1967; Brown and Brooks, 1974). Therefore, to get a complete picture of the me so-
pelagic fauna it will usually be necessary to use both small and large trawls and even in
that case biomass estimates based on trawls will at best be approximations.

Biological factors causing bias in net sampling

For ideal net sampling, we must know the biological or behavioural characteristics of
the target species, i.e. swimming behaviour, distributional pattern, development of sensory
organs, etc. These factors must be considered in order to decide the size, towing distance,
depth, speed, and structure of the net. But unfortunately, our knowledge of micronektonic
fishes is inadequate. The difficulty is that we must rely mainly on net sampling to study
the biological factors of the fish that bias the net samples. Problems of sample bias have
been reviewed by Harrison (1967) . The problems include the effect of the distributional
pattern and active avoidance of fishes in biasing the catch.

a) Pattern of distribution

Diel vertical migration; Diel vertical migration takes place in many species of meso-
pelagic fishes, and the pattern is species dependent (see section on diel vertical migra-
tion). Generally, the population density is higher at dawn, dusk and during the night than
during the day due to the greater vertical extension of distribution during the day. The
possibility that gravid females may be unable to take part in diel vertical migration was
pointed out for myctophids of the genus Diaphus (Nafpaktitis, 1968). This indicates that
we must change the towing depth and distance of the net in accordance with time or season
of sampling.

Ontogenic vertical migration; Almost all of the mesopelagic species change their
vertical distribution range during their life history, with younger individuals usually
inhabiting shallower depths (Badcock and Merrett, 1976). Therefore, wide vertical sampling
coverage is essential to collect samples representing the true age composition of the pop-
ulation in the sea.

Swarm, shoal, school and patch; Knowledge about schooling or other similar social
behaviour, such as size of the school, distance between schools and individual density is
necessary to decide sample size, since the net must filter a sufficient volume of water to
even out the irregularity of fish distribution (Harrison, 1967). This behaviour has been
observed through the window of deep-sea submersibles (Beebe, 1935; Peres et al.* 1957;
Barham, 1970, etc.), but present knowledge is still insufficient. Other patterns of irreg-
ular distribution due to physical phenomena such as currents, eddies and fronts are also
important factors influencing the sample bias.

Active responses of the fish to the nets

The problem can be largely divided into two elements: swimming behaviour and sensory
ability of the fish.

Swimming behaviour; In general, swimming speed is closely related to body size, shape
and feeding behaviour such as filter feeding, luring and darting. A functional morphological
approach will contribute greatly to learning about the swimming behaviour of mesopelagic
fishes at the present stage, where direct observation by deep-sea submersibles is insuf-
ficient and rearing techniques have not yet been established. Active swimmers will not only
'avoid the net, but also escape from the net mouth.

; Vertical orientation; Many individuals of the myctophids Lampanyctus leuoopsarus and
|. mexicanus, the bathylagid Bathylagus stilbius, the gonostomatid Cyclothone acclinidens>
| the chauliodontids, and the paralepidids have been reported to "hang" in the water at an

- 30 -

acute angle to the horizontal plane (Peres, 1958; Barham, 1970; see the review by Harrison,
1967). Barham (1970) reported that such inactive, vertically oriented fishes swim rapidly
away when stimulated and suggested that fishes actively feeding at night might be nearer
their exhaustion point and less capable of making a critical darting action to avoid the net
than inactive (resting) fishes during the day. Furthermore, it is possible that vertically
oriented fishes of the families Paralepididae and Stomiatidae more easily avoid nets by theii
upward darting behaviour than those oriented horizontally (Harrison, 1967).

Sensory ability; "The eyes of deep-sea fishes are probably by far the most sensitive
in existence" (Walls, 1942, cited by Marshall, 1954:221). This suggests the likelihood of
visual net avoidance by mesopelagic fishes, especially in the upper mesopelagic zone (200 -
500 m), during the day. The lateral line organ is also developed in many species and prob-
ably plays an important role in the perception of approaching nets. From the viewpoint of
functional morphology, a series of counter shading ventral photophores possessed by many
mesopelagic species probably indicates their vulnerability to attack from the ventral side.
In this context, Harrison (1967) discusses the towing methods of nets - vertical, oblique
and horizontal tows.

Much remains to be studied in the future concerning the senory ability to avoid nets.
The development of rearing techniques and more direct observations will be essential for
these investigations.

4.2 Acoustic survey^

Acoustics have been used intensively in fisheries science and, rather independently, by
investigators of the deep scattering layers (DSL), In fish stock assessment, medium frequen<
echosounders (38 - 50 kHz) and electronic echo integrators have been most commonly used. Th<
general description of the methods and the equipment used are given by Forbes and Nakken
(1972), and many of the problems involved are discussed by Margetts (1977). For studies of
DSLs low frequency echosounders (e.g. 12 kHz), or explosives and wide frequency range receiv-
ers (e.g. 0.5 - 25 kHz) have been commonly used, and the scattering strength as a function
of frequency has been recorded. A description of the methods and their application can be
found in Farquhar (1971) and Andersen and Zahuranec (1977).

Electronic echo integration at non-resonant frequencies

Generally a hull mounted transducer, an echosounder with a frequency of 38kHz or higher
and electronic integrators have been used, but they are now often replaced by digital integrators .

The fundamental principle of the integration method is: when a time varied gain (TVG)
compensating for one way geometrical spreading and two-way absorption of sound is applied,
and the voltage received from each echo is squared before integration, the output of the
echo integrator is linearly related to the biomass of fish per unit area in the sampled part
of the water column (see Forbes and Nakken, 1972) .

When fish length i much larger than wavelength, the relationship between integrated

echo abundance and fish density is well established (Nakken and Olsen, 1977). There is,

however, doubt about what happens when the fish length approaches wavelength (approx. 4 cm
for 38 kHz) .

For mesopelagic fish there is the additional problem that the proportionality coeffi-
cient for a given species is seldom known, and therefore it must usually be based on other
species. This may introduce bias, although it has been shown that the difference between
species decreases as the fish length approaches the wavelength (Nakken and Olsen, 1977) .

Another problem with the use of high frequencies is their rather short range. There-
fore, the TVG-f unction usually only covers the upper 500 m, and integration cannot success-
fully be carried out below that depth.

- 31 -

. Resonant frequencies

Scientists studying DSLs mainly use frequencies causing resonance in part of the popula-
tion studied. Often they use explosives to get a wide range of frequencies, and omnidirec-
tional hydrophones. Usually the working frequencies are between 2 and 15 kHz. The results
are usually given as scattering strength of the water column.

One fish giving resonance may contribute as much to the scattering strength as one
hundred non-resonant fish. Therefore, estimates based on these methods will in general only
include the resonant part of the population. How sharp the resonant peak and therefore the
size range giving resonance is, will depend on depth and on several anatomical features of
the swimbladder. Abundance estimates based on resonant frequencies can therefore be approx-
imations at best, and as they exclude fish not giving resonance, they will be consistently
too low.

For fish lacking a swimbladder, acoustical methods are generally of little use.

4.3 Egg and larva surveys

The errors in stock size estimates based on egg and larva surveys are mainly due to
sampling variance and insufficient information on biological factors. In regard to the
techniques, Smith and Richardson (1977) published a manual on standard techniques for pelagic
egg and larva surveys. Most of their techniques are readily applicable to egg and larva
surveys of mesopelagic fishes, although some modification would of course be needed in the
depth of sampling.

The main biological factors affecting the estimates are:

1. fecundity

2. sex ratio

3. mortality

4 . spawning area

5 . spawning season

As discussed in Section 3 on ecology and life history, information on these factors for
mesopelagic fish is poor at present, compared with that for commercially important epipelagic
species. And there are even more serious problems, such as insufficient knowledge on the
identification of eggs and larvae and their vertical distribution patterns.

Identification of most mesopelagic fish eggs has not been established, with the excep-
tion of some species such as LampanyatodeQ hectovis, Vinceguerria luoetia, Maurolicus muel-
leri and Chauliodus barbatus, etc. (Nishimura, 1957; Ahlstrom and Counts, 1958; Pertseva-
Ostroumova, 1973; Robertson, 1977). There have been numerous works on the development of
mesopelagic fishes (Myctophidae: TSning, 1918; Pertseva-Ostroumova, 1964, 1967, 1974; Moser
and Ahlstrom, 1970; Shiganova, 1977. Gonostomatidae: Ahlstrom and Counts, 1958; Kawaguchi
and Marumo, 1967; Ozawa, 1973, 1976; Okiyama, 1971, etc.). But in spite of these works,
the identification of mesopelagic fish larvae is still very difficult, owing mainly to a
, lack of practical keys to species. It should be stressed here that difficulties in ident-
; if ication are a great obstacle in the way of conducting quantitative egg and larva surveys
i of mesopelagic fishes.

Concerning the vertical distributional patterns of myctophid eggs, Yefremenko (1977)
reported myctophid eggs distributed in the 200 - 500 m layer in the Scotia Sea (near 57OS,
|30W to 60W), while Robertson (1977) reported the eggs of Lampanyctodes hectoris immediately
Sbeneath the sea surface. The occurrence of myctophid eggs in the upper mesopelagic zone in
Ithe Scotia Sea shows the need for sampling not only in epipelagic zone, but also in the
; mesopelagic zone below 200 m, during egg surveys of mesopelagic fishes.

Furthermore, there are indications that at metamorphosis some mesopelagic species
change their vertical range to depths below the usual daytime habitat of adults (Tftning,
1919; Jespersen and Tjfoing, 1926; Badcock and Merrett, 1976). These specimens at metamor-
phosis are essential to larva identification, since they have intermediate morphological
characters between postlarvae and adults. This is another reason for sampling in the meso-
pelagic zone during egg and larva surveys of mesopelagic fish.

- 33 -

5. NORTHEAST ATLANTIC

The mesopelagic fauna of the Northeast Atlantic Ocean is rather well known, at least
from a qualitative point of view. Most studies have, however, been carried out with gears
which - at best - should be considered semi-quantitative. Most data from the area are
derived from micronekton net collections. Acoustic studies, mainly at resonant frequencies,
are also important while data from commercial trawls and from egg and larvae surveys are
few. The life history of the few species which are abundant is rather well known.

Abundance

During August 1967, Zahuranec and Pugh (1971) (Table 5.1) occupied four stations in
the Norwegian Sea. Biological collections were made with a 6-foot IKMT fully lined with
1/4 inch knotted nylon netting. At their easternmost station (65N 0W) , 9 tows yielded
from to 3 x 10" 3 fish/m 3 . Assuming that the fish were distributed in the upper 500 m,
that the trawl had an 85% efficiency (Pearcy and Laurs, 1966), and that the mean weight of
the fish was 1 g, a mean biomass of about 2.4g/m 2 can be estimated. 97% of the catches were
Benthosema glaciate. At the more northern and western stations the catch rates were lower,
and about 0.6 and 0.1 g/m 2 were estimated for two stations at above 67N 4OW amd 68.3QON
8W respectively. No raesopelagic fish were caught at 69N 12W. The size of the B. glaoiale
ranged between about 20 and 65 mm with about 3 equal modes at 30, 41 and 50 mm.

In 1962 seven stations approximately on a line from about 45N 7W off the Bay of
Biscay to 36^N 14OW off southern Portugal was fished with a 1 600-raesh herring trawl (Krefft,
1974). The average catch was 5.6 fish/min. of trawling. Assuming that the efficiency of
the net was similar to that of a 100% efficient trawl with mouth-area 300 m 2 (see Section
4.1), and that the fish were distributed in the upper 1 000 m, this corresponds to a biomass
of about 0.2g/m 2 . As the size of the fish is not known, the mean weight is arbitrarily set
atlg. The best catch-rate obtained corresponded to about 1 kg/hour.

Kashkin (1974) occupied one station east of the Azores in June 1967 using a 10-foot
IKMT. He got catches about 4 x 10" 1 * to 3 x 10~" 5 fish/ra 3 in different depth strata. He
reported lengths of the fishes dominating in the different layers ranging from 30 - 40 mm
in the most shallow layer to a maximum of 120 mm in one of the deeper layers. He also
measured sound scattering at various frequencies and found good agreement between the trawl
catches and the acoustic estimates. Based on the data given, a biomass of about 0.5 g/m 2
can be estimated. The data refers, however, only to fish supposed to be important sound
scatterers, and therefore underestimates the total biomass.

Badcock and Merrett (1977) occupied three stations at 40N 53N and 60N along the 20
20W meridian in October 1970, May 1971 and April/May 1971 respectively. They fished hor-
izontal strata between 10 and 1 000 m using an RMT 8. At the northernmost station the bio-
mass was about 1.7 g/m 2 in the upper 1 000 m (assuming a mean weight for Cyalothone of 0.2g
and for other fish Ig). In number, Cyclothone made up about 50% of the catches. At 53N
and 40N the biomass was estimated to 1.5 and 1.1 g/ra , and the percentage of Cyelothone
about 60% and 80% in number.

Backus and Craddock (1977) published mean catch in ml/hr for faunal provinces in the
Atlantic. Their data are based on nighttime catches in the upper 200 m with a 10-foot IKMT.
Assuming that the efficiency of the gear is about 90% (Brooke et al^ 1973), that the fish
were evenly distributed in the upper 200 m and that there were no fish below this depth
during the night, the following results can be derived:

Atlantic Subarctic Region 1.2 g/m 2
Azores - Britain Province 0.1 g/m 2
Mediterranean Outflow Province 0.03 g/m 2

Vs the figures represent the upper 200 m only, they seriously underestimate the total biomass.

- 3? -

Furthermore, there are indications that at metamorphosis some mesopelagic species
change their vertical range to depths below the usual daytime habitat of adults (Tftning,
1919; Jespersen and T&ning, 1926; Badcock and Merrett, 1976). These specimens at metamor-
phosis are essential to larva identification, since they have intermediate morphological
characters between post larvae and adults. This is another reason for sampling in the meso-
pelagic zone during egg and larva surveys of mesopelagic fish.

- 33 -

5. NORTHEAST ATLANTIC

Abundance

During August 1967, Zahuranec and Pugh (1971) (Table 5.1) occupied four stations in
the Norwegian Sea. Biological collections were made with a 6-foot IKMT fully lined with
1/4 inch knotted nylon netting. At their easternmost station (65ON 0W) , 9 tows yielded
from to 3 x 1Q~ 3 fish/m 3 . Assuming that the fish were distributed in the upper 500 m,
that the trawl had an 85% efficiency (Pearcy and Laurs, 1966), and that the mean weight of
the fish was 1 g, a mean biomass of about 2.4g/m 2 can be estimated. 97% of the catches were
Benthosema glaciale. At the more northern and western stations the catch rates were lower,
and about 0.6 and 0.1 g/m 2 were estimated for two stations at above 67N 4ow amd 68.30N
8W respectively. No mesopelagic fish were caught at 69N 12W. The size of the B. glaciate
ranged between about 20 and 65 mm with about 3 equal modes at 30, 41 and 50 mm.

In 1962 seven stations approximately on a line from about 45N 7W off the Bay of
Biscay to 36N 14^W off southern Portugal was fished with a 1 600-mesh herring trawl (Krefft,
1974). The average catch was 5.6 fish/min. of trawling. Assuming that the efficiency of
the net was similar to that of a 100% efficient trawl with mouth-area 300 m 2 (see Section
4.1), and that the fish were distributed in the upper 1 000 m, this corresponds to a biomass
of about 0.2g/m 2 . As the size of the fish is not known, the mean weight is arbitrarily set
atig. The best catch-rate obtained corresponded to about 1 kg/hour.

Kashkin (1974) occupied one station east of the Azores in June 1967 using a 10-foot
IKMT. He got catches about 4 x 10~" to 3 x 10" 5 fish/m 3 in different depth strata. He
reported lengths of the fishes dominating in the different layers ranging from 30 - 40 mm
in the most shallow layer to a maximum of 120 mm in one of the deeper layers. He also
measured sound scattering at various frequencies and found good agreement between the trawl
catches and the acoustic estimates. Based on the data given, a biomass of about 0.5 g/m 2
can be estimated. The data refers, however, only to fish supposed to be important sound
scatterers, and therefore underestimates the total biomass.

Badcock and Merrett (1977) occupied three stations at 40N 53N and 60N along the 20
20W meridian in October 1970, May 1971 and April/May 1971 respectively. They fished hor-
izontal strata between 10 and 1 000 m using an RMT 8. At the northernmost station the bio-
mass was about 1.7 g/m 2 in the upper 1 000 m (assuming a mean weight for Cyelothone of 0.2g
and for other fish Ig). In number, Cyclothone made up about 50% of the catches. At 53N
and 40N the biomass was estimated to 1.5 and 1.1 g/m , and the percentage of Cyelothone
about 60% and 80% in number.

Backus and Craddock (1977) published mean catch in ml/hr for faunal provinces in the
Atlantic. Their data are based on nighttime catches in the upper 200 m with a 10-foot IKMT,
Assuming that the efficiency of the gear is about 90% (Brooke et al.> 1973), that the fish
were evenly distributed in the upper 200 m and that there were no fish below this depth
during the night, the following results can be derived:

Atlantic Subarctic Region 1.2 g/m ?
Azores - Britain Province 0.1 g/m 2
Mediterranean Outflow Province 0.03 g/m 2

ks the figures represent the upper 200 m only, they seriously underestimate the total biomass.

- 34 -

Table 5.1
Abundance estimates derived from trawl surveys in the Northeast Atlantic

Area

Gear

Author

Estimated
biomass g/zn 2

Norwegian Sea 65N 0W
67N 4W

IKMT 6
IKMT 6

Zahuranec and Pugh, 1971
Zahuranec and Pugh, 1971

2.4
0.6

68.30N 8W

IKMT 6

Zahuranec and Pugh, 1971

0.1

69N 12W

IKMT 6

Zahuranec and Pugh, 1971

E. Azores 41N 14W

IKMT 10

Kashkin, 1974

0.6

40N 20W

RMT 1+8

Badcock and Merrett, 1977

1.1

53N 20W

RMT 1+8

Badcock and Merrett, 1977

1.5

60N 20W

RMT 1+8

Badcock and Merrett, 1977

1.7

Biscaia -
West Portugal

Engel trawl

Krefft, 1974

0.1

Kashkin (1967) has presented data on biomass of mesopelagic fish based on catches
obtained in various micronekton nets on a number of cruises. His data are given in g/m 3
and arbitrarily assuming that they represent the mean of the upper 1 000 m they can be con-
verted to g/m 2 surface area. For the Norwegian Sea he got about 0.3 g/m 2 . A few hauls west
of Iceland yielded about 2 g/m 2 . In the best sampled part of the area, west of the British
Isles, he got about 0.8 g/m , and in the east of the Northwest Atlantic between 0.1 and 0.8
g/m 2 .

During 1971 and 1972 Williams and Hart (1974) studied the distribution of fish eggs
and larvae at 59N 19W. Maupolicua muelleri was by far the most important species. Its
eggs were abundant from May through August. For both years the mean number of eggs per m 2
surface area was about 100. In 1971 the spawning reached a peak in June when 400 eggs/m 2
were observed. In 1972, 500 eggs/m 2 were taken during the same month. A minimum estimate
of the spawning stock can be obtained by considering only the peak of eggs abundance. Assum-
ing a fecundity of 300 (Gj^saeter, 1978), a sex ratio 1:1 and a mean weight of 1.2 g (l45mm) ,
4 g/m 2 of fish must have contributed to the peak in 1972 and 3 g/m 2 in 1971.

When the incubation time is known, the egg abundance distribution may be integrated to
get the total number of eggs spawned during a spawning season, and from this the spawning
stock can be estimated. If it is assumed that the eggs of MaurolicuB hatch after 10 days
and that each female spawns once during the spawning season, an estimated spawning stock
size of 12 g/m 2 can be derived for each of the two years.

Larvae of various Myctophidae were also caught, but no data suitable for abundance
estimation were reported.

- 35 -

45 W

Fig. 5.1 Stations where Chapman ct al. (1975) made the
acoustic measurements referred to in Table 5.1

Gj^saeter (1978) estimated the abundance of mesopelagic fish off the British Isles and
southern Norway using echosounders and electronic integrators. In Norwegian waters where
Maurolicus muel'leri was the dominant species, stock size estimates within the area covered
ranged from 2 000 to 1 600 000 tonnes. (On a large part of the cruises the area of distribu-
tion was only partly covered). Generally the biomass was 10 to 35 g/m ? although the highest
average observed over 5 n miles was 160 g/m 2 .

West of the British Isles the area from about 51N was covered during early spring 1972
1973 and 1974. Due to problems with the identification of scatterers and insufficient know-
ledge about their scattering characteristics only tentative abundance estimates can be de-
rived. If it is assumed that M, muelleri was the only mesopelagic fish the biomass in the
area covered was about 5, 0.6 and 2 million tonnes in 1972, 1973 and 1974 respectively, or
between 6 and 30 g/m 2 . An increasing proportion of the two other important species in the
area, Benthoserna glaciate and Notoscopelus kroeyeri, will give higher estimates. Assuming
N. kroeyeri to be the only scatterer will give 13, 2 and 5 million tonnes or more for the
three years.

Acoustic studies covering a wide range of frequencies have been reported by Chapman et
al. (1975), and some of their results from the Northeast Atlantic are summarized in Table 5.2
together with the estimates derived from them. Two stations south of Iceland gave biomass
estimates of 1.6 and 0.7 g/m 2 respectively, and two stations in the south-western part of
the region gave 1.3 and 1.8 g/m 2 . As only fish giving resonance at the frequencies involved
will contribute to these values, they will probably be serious underestimates of the true
biomass of mesopelagic fish.

- 36 -

Synthesis of abundance estimates

Most of the biomass estimates presented in Table 5.1 are underestimates as the catch
rates of micronekton nets were used without considering avoidance, which is probably the
most important source of bias (e.g. Scully-Power, 1977). The estimates based on acoustic
surveys using resonant frequencies are also underestimates as non-resonant fish will con-
tribute very little to the values obtained. The estimate based on egg samples referred to
only one species, and therefore underestimates the total biomass. The data obtained by
acoustics at non-resonant frequencies may be over- or underestimates of the true biomass.

It is difficult to find geographical trends in the biomass. There seem, however, to
be fairly high concentrations in a zone between western Norway and southern Greenland where
about 2 g/ra 2 seems to be a conservative estimate. From this zone there seems to be a de-
crease to the north and to the south. The biomass seems to be very low in the southeastern
part of the area where values of about 0.5 g/m 2 were obtained. This is consistent with the
observation of Chapman et al. (1977) that there was a decrease in scattering strength from
the Azores to Gibraltar. The data given by Chapman et a. (1975) and Backus and Craddock
(1977) indicate an increase in biomass in the western part of the area. Probably the bio-
mass there is higher than 2 g/m 2 .

The highest concentrations of mesopelagic fish are, however, found in the neritic areas
off southern Norway and to the west of the British Isles. The egg samples from 59N 19W
indicate that this concentration may extend fairly far offshore. There is little doubt that
the mean biomass in these areas is at least of the order of 10 g/m 2 . It is not known whether
similar concentrations are found near Iceland and southern Greenland.

Table 5.2

Abundance estimates derived from acoustical data
presented by Chapman et al., 1975

St. No.

Scattering
Strength

N/m 2

L mm

Biomass
g/m 2

0.13

-44

1.33

0.46

0.6

0.32
0.18

-44
-48

0.22
0.28

93
52

8.5
1.5

1.9 )
) 2.3
0.4 )

0.24
0.31

-44
-44

0.39
0.23

70
90

3.7
7.8

1.4 )
) 1.6
1.8 )

0.11

-45

1.47

0.25

0.4

no peak

R is swimbladder radius (cm); N is estimated number of fish; L and W are estimated
length (mm) and weight (g) of the resonant fish. For distribution of the stations
see Fig. 5.1.

- 37 -

Table 5.3 gives estimates of the stock of mesopelagic fish in the subareas shown in
Fig. 5.2. Except for subarea 5 the estimates are based on catches in micronekton nets. For
area 5, a mean of the estimates given by Gjtfsaeter (1978) is used. For the waters west of
the. British Isles the lowest estimates were taken, on the supposition that M. muell&ri was
the only sound scatterer .

Table 5.3

Abundance estimates for various parts of the Northeast Atlantic

For areas see Fig. 5.2.

Area No .

Size of Area
m 2 x 10 11

Biomass g/m ?

Stock
tonnes x 10 6

7.4

0.1

0.07

8.8

0.5

0.44

8.6

2.0

1.71

12.4

1.5

1.86

3.00

18.9

1.1

2.08

11.2

0.5

0.58

25.0

2.0

5.00

^ 92

14.74

ft5N

__, . , ,_ --. , . '30

5 6W 46 3 S 10 10 20 30

Fig. 5.2 Subareas of the Northeast Atlantic used in the biomass
assessment (Table 5.3), The smaller figures indicate
mean biomass in g/m

Depth distribution

Depth distribution of the DSLs in the Northeast Atlantic was studied by Haigh (1971)
using a 10 kHz echosounder. In the Norwegian Sea and Irminger Sea the depth of the only
layer found varied from about 250 to 350 m. Between 50 and 60N the main layer was observed
at depths from about 300 to 400 m and in parts of the area an additional layer was observed
at about 550 m. Between 35 and 50N the main layer was usually observed between 350 and
600 m. Additional layers were sometimes observed both above and below these depths. The
deepest sometimes extending down to more than 800 m.

Badcock and Merrett (1977) give data based on trawl catches indicating that at 60 the
catches were rather evenly distributed between 300 and 1 000 m. At 53N a maximum was re-
corded at 500 m depth and at 40N high catch rates were obtained between 400 and 600 m and
at 700 m. At the two northernmost stations good catch rates were obtained in the upper
200 m during the night. At 40N they found little evidence of vertical migration.

Species composition

Hureau and Monod (1973) give a comprehensive and up-to-date list of mesopelagic fish
found in the Northeast Atlantic. The families Myctophidae, Gonostomatidae and Sternopty-
chidae totally dominate the mesopelagic fauna in the area. For purposes of identification
Nafpaktitis et al. (1977) should be used for the Myctophidae. For the Gonostoraatidae, Grey
(1964) should be consulted and for the Sternoptychidae, Baird (1971).

In most of the Northeast Atlantic Benthosema glaciate is a dominant species. It is
found off south Greenland, Iceland, Spitsbergen and in the western Barents Sea in the north.
To the south its distribution extends to the border of the area (Bolin, 1959; Bekker, 1967).
It is also abundant in the deep fjords of western Norway (Gj^saeter, 1973) .

MauroHcus muelleri is also very important in the north Atlantic. In neritic areas
off southern Norway it is the most important species, and probably it is one of the most
abundant species west of the British Isles (Gjdsaeter, 1978). In the north it ranges to
the north of Norway and Iceland. In the south it is found beyond the border of the area
(Grey, 1964).

Off Great Britain Notoscopelus kroeyeri is abundant although the population appears to
be expatriated (Gjrfsaeter, 1978). Its general range is from about 65N to about 37N
(Bolin, 1959; Kashkin, 1974).

In the southern part of the area the species Lobianchia dofleini, Lanrpanyctus pusillus,
Cera tosoope lus maderensis and Argyropelecus olfersi are also of some importance (Geistdoerfer
et al., 1971; Kashkin, 1974; Harrison, 1967; Backus et al. 3 1977).

Cyolothone seem to be an important component of the oceanic mesopelagic fauna, but due
to their very small size (usually less than 30 mm) and their deep distribution they are not
of much interest from a practical point of view. The dominant species in the Northeast
Atlantic are C. microdon and C. braueri (Badcock and Merrett, 1977). For a more detailed
discussion of the zoogeography of the mesopelagic fish in the Northeast Atlantic, Backus
et al. (1977) should be consulted.

Life history

Benthosema glaoiale is one of the best known mesopelagic fish species as far as life
history is concerned (Halliday, 1970; Gjrfsaeter, 1973, 1973a, 1978). It seems to have a
highly variable growth rate, even within the northern part of the area studied, K from the
von Bertalanffy growth curve ranging from 0.19 to 0.46 and L^ from 70 to 87 mm. The natural
mortality is about 0.7. Sexual maturity is reached after an age of 2 or 3 years and the
mean fecundity is about 700 eggs per female. In the southern part of the Northeast Atlantic
area the life cycle seems to be shorter and the maximum size smaller.

- 39 -

It feeds partly on copepods and partly on euphausids and belongs therefore partly to
the first and partly to the second carnivorous level (Gjrfsaeter, 1973a) .

Notoecopelus kroeyeri was studied by Gjdsaeter (1978) , Its growth can be described by
the equation

1 119 nro (1- exp [-0.89(t + 0.17)])

and the mortality in the adult population is about 0,8 west of the British Isles, N kroeyeri
seems to be expatriated, and it is not known where it spawns. The main food is euphausids
but some copepods are taken.

Maurolicus muelleri was studied by Gjrfsaeter (1978), Its growth can be described by
the equation

l t 59 mm (1- exp [-0.88 (t + 0.06)])

and the mortality rate is about 1.8. Spawning may take place when the fish is one year old,
and the number of ripening oocytes per female ranges between 200 and 500. The young fish
feed mainly on copepods while copepods and euphausids were about equally important in adult
fish.

Aspects of the life history of Cyclothone which seem to be important in the oceanic
parts of the area have been studied by Badcock and Merrett (1976). Some of their results
are presented in Section 7 (Eastern Central Atlantic, where their studies were carried out) .

- 41 -

6. NORTHWEST ATLANTIC

The mesopelagic fauna of the Northwest Atlantic Ocean is rather well known, at least
from a qualitative point of view. Most studies have been carried out using micronekton nets,
but there are also some data based on direct observations from submersibles and acoustic
(resonant frequency) studies. Quantitative estimates based on these methods seem to be di-
vergent, the direct observations from submersibles giving the highest fish densities.

Abundance

The most extensive micronekton net sampling in the Northwest Atlantic was reported by
Backus et al. (1970) and Jahn and Backus (1976). Jahn and Backus (1976) made 51 collections,
partly by a half-lined 10-foot IKMT and partly by fully-lined nets. To make the two sets of
collections comparable, those taken by the half-lined nets were multiplied by 1.9. Only
nighttime catches in the upper 200 m were used.

Assuming an efficiency of 90% for the gear, the following results are derived:

Slope water 0.2 g/m 2

Gulf Stream 0.1 g/m 2

North Sargasso Sea 0.05 g/m 2

Backus et al. (1970), using the same methods, got about 1.7 g/m 2 for the Labrador region.
All these figures represent the upper 200 m only, and they therefore seriously underestimated
the total biomass, especially in the southern areas, where Cyclothone and others which do not
migrate to the upper 200 m during the night are important.

Kashkin (1967) has presented data on bioraass of mesopelagic fish based on catches ob-
tained in various micronekton nets on a number of cruises. His results are presented in g/m 3
and assuming that the haul represents the mean fish density in the upper 1 000 m they can be
converted to g/m 2 surface area. Off southeast Greenland he found about 8 g/m 2 based on 24
hauls. The rest of the area was only poorly sampled, but a much lower biomass (0.3 - 0.7
g/m 2 ) was indicated.

Acoustical studies covering a wide range of frequencies have been reported by Chapman
et al. (1975). Some of their results and estimates based on them are presented in Table 6.1.
The positions of the stations are shown in Fig. 6.1. A station off southeast Greenland
(No. 4) gave 4.7 g/m 2 of a scatterer with length about 10 cm. All other stations except one
in northern Baffin Bay (No. 4) where 9 cm scatterers amounted to about 3.6 g/m 2 , gave values
below 1 g/m 2 and usually below 0.5 g/m 2 .

Johnson et al. (1956) made a study with a suspended echosounder off the continental
slope south of New England (39 34'N 70o 32'W) , The experiment was carried out during the
night and the density in the surface layer was too high to be recorded. Between about 70
and 600 m a mean density of 7 x 10" 1 * fish/m 2 was observed. Assuming that the density in the
upper layer is twice that recorded between 70 and 100 m, and that the mean weight of the
targets was 1 g, this corresponds to a biomass of about 0,5 /m 2 surface area*

Backus et al. (1968) observed a peculiar sound-scattering-layer at about 40N 70W, and
observations from a submersible showed that it consisted of schools of Ceratoscopelus maderen-
sis. The schools were observed at depths from about 330 m downwards during the day and as
shallow as 20 m during the night. This layer was observed in all parts of the slope water,
but was irregularly developed. A medium sized school (25 m diameter, 7 m thick) contained
about 40 000 fish. The mean size of the fish was about 6 cm corresponding to about 2.4 g.
The weight of the fish in one school is therefore about 100 kg. The number of schools was
estimated to be about one per 238 x 10 5 m 3 using echo sounders from surface vessels and one
per 7.5 x 10 5 m 3 using sonarscopes in the submersibles. Taking a mean of these observations
and assuming that the schools are distributed in a layer 200 m thick (see Fig. 1 of Backus
et al. 9 1968), the biomass in the area was about 13 g/m 2 .

- 42 -

Table 6,1

Abundance estimates derived from acoustic data
presented by Chapman et al. 9 (1975)

St.

Scattering
Strength

N/m 2

Biomass
g/m 2

0.10

-43

2.7

0.2

0.5

0.16
0.08

-46
-43

0.5
4.2

46
23

0.9
0.1

0.5 )
) 0.8
0.3 )

0.13

-47

0.6

0.5

0.3

0.36

-40

0.4

104

11.9

4.7

0.13
0.10

-47
-47

0.6
1.1

38
29

0.5
0.2

0.3 )
) 0.5
0.2 )

0.11

-50

0.4

0.3

0.1

0.08

-50

0.8

0.1

0.32

-41

0.4

9.1

3.6

R is swimbladder radius (cm): N is estimated number of fish; L and W are estimated
length (mm) and weight (g) of the resonant fish. For distribution of the stations
see Fig. 6.1.

Milliman and Manheim (1968) made a dive with the submersible Alvin northeast of Cape
Hatteras (35 37 f N 74o 49'W) in July 1967. They observed concentrations of myctophid-like
fishes exceeding 0.1 per m 3 between 300 and 340 m and more than 0.5 per m 3 at about 570 m.
Based on their Fig. 2 an abundance of about 100 myctophid-like fish/m surface area can be
estimated. The size of the fish was 2 - 6 cm corresponding to a weight about 0.04- 2.5g.
Assuming a mean weight of 0.6 g (1 * 4 cm) this corresponds to 60 g/m 2 .

Synthesis of abundance estimates

It is difficult to synthesize the estimates available from the Northwest Atlantic as
the data are very divergent. It seems that fairly good concentrations are found off south-
east Greenland where estimates between about 5 and 8 g/m 2 have been obtained. There are
also indications that the slope water off Canada and USA holds a higher biomass than the
Gulf Streamwater and the north Sargasso Sea. Although the samples with micronekton nets gave
between 0.05 and 0.2 g/m 2 for these areas, acoustic studies indicate that the true values
must be considerably higher, at least 0.5 g/m 2 . Direct observations indicate values as high
as 13 g/m 2 to 60 g/m 2 in the slope waters, but the variation of these concentrations in area
and in time is not known.

- 43 -

-60

-50

Fig. 6.1 Stations where Chapman et al. (1975)
made the acoustical measurements
referred to in Table 6.1

96 w 75

65N

Fig. 6.2 Subareas of the Northwest Atlantic
used in the biotnass assessment
(Table 6.2). The smaller figures
indicate mean biomass in g/m

5Qo ^0

-30
30

- 44 -

With the information available, the figures shown in Table 6.2 can be estimated (for
areas used see Fig. 6.2). The estimate for area 4 may be high. The other estimates are
probably conservative.

Based on these calculations the stock size in the Northwest Atlantic is about 15 million
tonnes, similar to that in the northeast part of the ocean.

Table 6.2

Abundance estimates for various parts of the Northwest Atlantic

For areas see Fig. 6.2.

Area No.

Area
m 2 x 10 11

Biomass
g/m 2

Stock
tonnes x 10 6

3.0

3.6

1.1

1.6

0.6

1.6

6.5

1.0

26.0

1.7

4.4

7.2

10.0

0.5

14.8

Depth distribution

The depth distribution of the DSLs in the Northwest Atlantic was studied by Haigh
(1971). Using a 10 kHz echo sounder he observed one DSL at about 350 m in the western part
of the area and at about 400 - 450 m in the eastern part. Backus et at. (1968) observed
that the schools of Ceratoscopelus found in slope waters were distributed below 330 m during
the day and up to 20 m during the night. Off Nova Scotia, Halliday (1970) observed that the
most important mesopelagic fish in that area, Benthosema glaciate, had a daytime distribu-
tion with centre below about 450 m (upper extension 150 m) and a nighttime distribution
centred between 45 and 90 m.

Species composition

The families Myctophidae, Gonostomatidae and Sternoptychidae dominate the mesopelagic
fauna. For identification, Grey (1964) and Baird (1971) are useful for Gonostomatidae and
Sternoptychidae respectively. For Myctophidae, Nafpaktitis et al. (1977) give a comprehen-
sive review with keys to genera and species. More details about the zoogeography of the
mesopelagic fishes are given by Backus et al. (1977).

The relative importance of the different species has been discussed by Backus et al.
(1970) and Jahn and Backus (1976) and some of their results are presented in Table 6.3.
These studies were, however, based on night hauls in the upper 200 m and non-migrants such
as Cyclothone etc., are therefore excluded.

- 45 -

Table 6.3

Relative importance of the most important mesopelagic
fish species in the Northwest Atlantic (data from Backus
et al. (1970) and Jahn and Backus (1976))

Species

Labrador
area

Slope
water

Gulf
Stream

Northern
Sargasso Sea

Benthosema glaciale

Ceratosoopelus maderenais

C. warmingi

Diogenichthys atlanticus

Lopidophanes guentheri

Lobiancia dofleini

Notolyohnus valdiviae

Stomias boa

Life history

Benthosema glaciate has been studied by Halliday (1970) who found that the growth can
be described by the equation

1 - 68.28 (1 - exp [- 0.36 (t + 0.49)])

Other aspects of the life history also seem similar in the western and eastern North
Atlantic (Gjrfsaeter, 1973, 1978).

Lobianoia dofleini from Bermuda waters was studied by Karnella and Gibbs (1977). They
found that it reaches a maximum size of about 38 mm, spawns mainly in winter, and has a one-
year life cycle.

Notolyohnus valdiviae, studied by Gibbs et al. (1971), is a very small species. The
maximum size in their samples was 22 mm. The species apparently has a one-year life cycle
and the spawning seems to increase in intensity from spring to a peak in early summer.

Ceratoseopelus warmingi seem to reach sexual maturity at a length of 40 mnu and the
maximum size can be more than 80 mm (Bekker and Borudinula, 1968). In the Bermuda area
spawning seems to peak in June or July (Gibbs et al. 9 1971). In Hawaii waters the species
seem to spawn over a long period, but principally during the first half of the year (Clarke,
1973).

- 47 -

7. EASTERN CENTRAL ATLANTIC

From the Eastern Central Atlantic there are data obtained by micronekton nets, commer-
cial trawls, acoustic surveys and larval sampling. For the offshore areas it seems possible,
therefore, to obtain a consistent, although conservative, estimate of the biomass. For the
coastal waters there are indications of large stocks but more research has to be carried out
to obtain a better picture of these concentrations and their variation.

Abundance

Badcock and Merrett (1976) occupied a station at 30N 23W during April 1972, fishing
horizontal strata between 10 and 1 500 m using an RMT 8. Assuming that the gear has an ef-
ficient opening of 8 m 2 , two series of 14 day hauls and 14 night hauls both indicate a
density of between 9 and 10 fish/m 2 (Maoror'amphosus soolopax excluded). Of this, Cyolothone
spp. made up nearly 70%. If the weight of Cyelothone is arbitrarily set at 0.2 g and that of
the other mesopelagic fish at 1 g, the biomass is about 4 g/m 2 .

Badcock and Merrett (1977) reported similar stations at 18N 25W and at 11N 20W, but
only the water down to 1 000 m was sampled. On the same assumptions as above the following
results are obtained.

Position Fish/m 2 Percent Cyelothone Biomass g/m 2

18N 25W 11 65 4.1

11N 200W 15 76 6,0

In the period 1966-71 Krefft (1974) made several cruises fishing with a 1 600-mesh
herring trawl. Most of the stations in the Central Atlantic fell approximately on a line
35N 18W to 0N 23W. The mean catch was about 26,5 fish/min. of trawling. Assuming that
the efficiency of the net was similar to a 100% efficient trawl with mouth area 300 m (see
Section 4.1), that the towing speed was 3.5 knots, that the fish were evenly distributed in
the upper 1 000 m and that the mean weight was about 1 g/fish, this corresponds to a biomass
of about 0.8 g/ra 2 .

Blackburn (1977) reported 11 hauls with a micronekton net (type not specified) just
beyond the edge of the shelf at about 22N. He found a fish biomass of about 0.2 g/m 2 .

Voss (1969) described the mesopelagic fauna in the Gulf of Guinea. Although no quan-
titative data are presented, he states that the fauna is very rich, and that IKMT tows gave
about ten times higher biomass than comparable tows in the Caribbean Sea.

Kashkin (1967) presented data on the biomass of raesopelagic fish based on catches ob-
tained in various micronekton nets on a variety of cruises. His data is given as g/m 3 and
assuming that the hauls represented the mean of the upper 1 000 m they can be converted to
g/m 2 surface area. Twentyfive hauls off West Africa north of 10N gave a mean of 0.6 g/m 2
and 26 hauls south of 10N and in the Gulf of Guinea gave 4.3 g/m 2 .

Backus and Craddock (1977) published the mean catch in ml/hr for faunal provinces in
the Atlantic. Their data are based on nighttime catches in the upper 200 m with a 10-foot
IKMT. Assuming that the efficiency of the gear was about 90% (Brooke et al.> 1973), that
the fish were evenly distributed in the upper 200 m, and that there were no fish below this
depth at night, the following results can be derived:

North African Subtropical Sea 0.1 g/m 2
Mauritanean Upwelling Region 0.2 g/ra 2
Guinean Province 0.2 g/m

- 48 -

Gjrfsaeter and Blindheim (1978) estimated the abundance of raesopelagic fish off West
Africa between 16 and 27N based on an acoustic survey during November-December 1972. They
used a 38 kHz echo sounder and electronic echo integrators. The equipment was calibrated
using a 1 600-mesh pelagic trawl. Due to the low efficiency of the trawl, this procedure
will usually give underestimates of the true biomass. They also calculated the biomass by
using a conversion constant between integrated echo intensity and biomass derived forcapelin.

Based on the calibration by trawl they found a biomass of about 6 million tonnes in the
area. The highest biomass was observed just off the edge of the continental shelf where a
mean biomass of 60 g/ra 2 was observed. In offshore waters the mean biomass was 15 g/m 2 .
Using the density coefficient based on cap el in these estimates will be about 3 times higher.

A typical distribution of biomass along a section normal to the shore is shown in
Fig. 3.3, From 27N there was a gradual increase in biomass until Cape Blanc where the
highest biomass was observed. Southward the density decreased.

Little trawling was carried out in the mesopelagic fish layer, but in one tow a catch
rate of 6 tonnes/hr was obtained. A tow with 0.6 tonnes/hr ranked next. Diaphus dumerili
was dominant in the catches.

Several studies of fish eggs and larvae have been carried out in the region (Table 7.1).
Nellen (1973) studied an area near the Great Meteor Seamount. In oblique tows with a ring
trawl down to 200 m he got about 6 larvae/m 2 of each of the families Gonostomatidae and
Myctophldae. These made up about 42% of the total larvae caught. Among the Gonostomatidae,
Cyclothone spp. represented about 73%.

Blackburn and Nellen (1977) studied the nearshore ichthyoplankton at about 22N. Mauro-
Hcus rmelleri was abundant at the shelf edge (100 eggs/m 2 in part of the area), but the
mean in the whole area studied was only 17 eggs and nearly 2 larvae/m 2 . Myctophidae represent-
ed only one larva/m 2 . The mesopelagic fish represented 2.6% of the total fish larvae caught.

Palomera and Rubies (1978), studying a nearshore area between about 23 and 26N found
a mean of about 66 eggs and 20 larvae of Maurolicus muelleri per m 2 in stations taken near
the edge of the shelf. In the same area they caught 7 myctophid larvae per m 2 .

In an area between 10 and 17N and from the coast to 22W, Aboussouan and Aldebert
(1978) caught a mean of 26 Myctophidae and 12 Gonostomatidae larvae per m 2 . The mesopelagic
fish made up 74% of all the larvae caught.

As little is known about seasonal variation it is difficult to draw conclusions about
adult biomass from larval abundance.

Synthesis of abundance estimates

There seems to be little doubt that the Eastern Central Atlantic is a rich area as far
as biomass of mesopelagic fish is concerned. Kashkin (1967) and Backus and Craddock (1977)
published results suggesting that the Gulf of Guinea area is among the richest in the
Atlantic. It also seems clear that there is a very high biomass in the upwelling area off
West Africa. It is not known whether comparable densities may be found in the highly pro-
ductive parts of the Gulf of Guinea.

The USSR fishing fleet is catching myctophids off West Africa, and according to a
questionnaire sent out by FAO, they regard both Myctophidae and Gonostomatidae as potential
resources in the Eastern Central Atlantic (W. Fischer, pers. com.). No further information,
however, seems to be available about this fishing.

- 49 -

Table 7.1
Number of eggs (E) and larvae (L) of mesopelagic fish per

in the Eastern Central Atlantic. The number of mesopelagic fish larvae
as percentage of all the larvae caught is also given

Area

Period

Gonostom.
E L

Myct.
L

Mesopel.

Author

29-31'N 27-30'W

February 1970

42%

Nellen 1973

22N 17-18W

March-May 1974

17 2

2.6%

Blackburn and
Nellen 1976

10-17N 17-22W

June-July 1973

74%

Aboussouan and
Aldebert 1978

23-26N 14-17W

April-May 1973

66 20

Palomera and
Rubies 1978

Table 7.2

Abundance estimates for various parts of the Eastern Central Atlantic
(Numbered areas shown in Fig. 7.1)

Area
No.

Area size
m 2 x 10 11

Bioraass
g/m 2

Stock
tonnes x 10 6

133

- 50 -

There are, however, few data for estimating absolute abundance. For the northern and
western parts of the area the estimates (Table 7.2, Fig. 7.1) are based on micronekton net
hauls carried out by Badcock and Merrett (1976, 1977). As there is much evidence that the
biomass in the Gulf of Guinea is higher than off the west coast, the biomass there is arbi-
trarily set at 6 g/ra 2 . For the Mauritanian upwelling area the most conservative estimate
of offshore biomass (15 g/m 2 ) derived by Gj^saeter and Blindheim (1978) is used. The very
high concentrations of mesopelagic fish observed over the shelf edge are not included in
Table 7.2, as they may be temporary phenomena. If they are more stable, and similar concen-
trations are also found in the upwelling area in the Gulf of Guinea, even the most conserva-
tive estimates presented by Gjrfsaeter and Blindheim (1978) suggest that these concentrations
alone may amount to about 6 million tonnes or more.

The data on eggs and larvae are not used in this assessment but the ratio between num-
ber of larvae observed and the estimated biomass does not seem to conflict with what is known
about this ratio in other areas.

Depth distribution

Haigh (1971) studied the depth-distribution of the DSLs in the area using a 10 kHz echo-
sounder. In most of the area he found two layers. The most important one had a daytime
depth of about 500 - 600 m in the northern and western part, and about 450 m in the more
productive upwelling areas. The second layer was observed between about 250 and 400 m.

Badcock (1970) and Badcock and Merrett (1976, 1977) made detailed observations on ver-
tical migration in offshore waters using micronekton nets. They found that during the day
the best catches (in number of specimens) were made between 400 and 700 m. The most impor-
tant Myctophidae were generally caught between 400 and 700 m during the day and from 10 to
100 during the night.

In the Mauritanian upwelling region Kinzer (1977) observed a DSL varying in depth be-
tween 80 and 200 m during the day, while Benthosema glaciale, which was the dominant species
in his RMT 8 catches, was mostly aggregated between 150 and 400 m. Gjrfsaeter and Blindheim
(1978) using a 38 kHz echo sounder found the main DSLs at a depth between 300 and 500 m
during the daytime in the same area. Parts of these layers migrated upwards and made very
dense concentrations near the surface during nighttime.

Species composition

The mesopelagic fish fauna of the Eastern Central Atlantic is very rich. For identifi-
cation of myctophids, Nafpaktitis et al. (1977) should be consulted. For storaiatidiform
families Grey (1964) and Baird (1971) are useful. A useful checklist is that of Kotthaus
(1972).

At a station at 3QQN 23W, Badcock and Merrett (1976) identified 37 families, 66 genera
and 98 species. The Myctophidae with 15 genera and 31 species, and the Gonostotnatidae with
7 genera and 17 species, were dominant. At 18ON still more species were identified (Badcock
and Merrett, 1977) .

Table 7.3 shows the 5 species ranking highest in number at stations at 30N 23W and
off Fuerteventura. Only small species are among the highest ranking, and this may be an
effect of the small gears used. Catches with an Engel trawl in the same area were dominated
by the myctophids which represented 88% of the fish taken (Harrison, 1967).

Plankton samples from the same general area (Nellen, 1973) gave the following ranking
among the mesopelagic fishes:

1. Cyclothone spp.

2. Hygophum spp.

3. Vinciguerria spp.

4. Lobianehia spp.

- 51 -

-0

Fig. 7.1 Subareas of the Eastern Central Atlantic used
in the biomass assessment (Table 7.2). The
smaller figures indicate mean biomass in e/ m

In nearshore waters between about 23 and 26 N Palomera and Rubies (1976) found larvae
of the Gonostoraatidae Maurolicus muelleri and Vinciguerria spp. and the Myctophidae Cera-
tosoopelus maderensis, Notosoopelus spp., Diogenichthya atlanticus, Uygophum re^nhardt^,
Lobianahia gemellari, Myctophum punctate, Lampanyahtus sp. and Benthosema glacvaie to_be
abundant. Kinzer (1977) found Benthosema glaciate to be far the most abundant species in
IKMT catches from this area. Samyshev and Schetinkin (1971) caught Dzaphue dum&nli, D.
taaningi, -Lepidophanes gu&ntheri and Maurolioua mueller-i in pelagic trawls. Cjrfsaeter and
Blindheim found Diaphus dwnerili to be dominant in catches by large pelagic trawls followed
by Myctophum punatatim and Diaphus taaningi. According to Backus et al. (1977) Dijpftwo
holti, Lepidophanes guentheri and Lampanyctus pus^^lus were most abundant in ILKI hauls.

For a more detailed discussion of the zoogeography of the mesopelagic fish in the area,
Backus et al. (1977) should be consulted.

- 52 -

Table 7.3

Ranking of the most abundant mesopelagic fish species
in offshore waters off Northwest Africa

Species

30N 23W
Badcock and Merrett 1976

Off Fuerteventura
Badcock 1970

Cyolothone braueri
C. nricTodon

Vinoiguerria tripunctulatus
Agyropeleous hemigyrmus
Sternoptyx diaphana
Benthosema suborbitale
Lobianchia dofleini
Lampanyctus pusillus

1
2
4
3

1
3

4
5

Life history

The life history of Cyolothone brauevi was studied by Badcock and Merrett (1976). The
species is sexually dimorphic with large females and small males. The size distribution of
males was unimodal with a peak at 16 - 18 mm and that of the females was bimodal with peaks
at 16 - 18 mm and 23 - 26 mm. Maximum size is about 38 mm. The female matures at a size
of about 22 mm and probably spawns only once, at an age of two years. Probably spawning
occurs in the spring. The fecundity varies between about 200 - 400 eggs.

Badcock and Merrett (1976) also studied the life history of Cyolothone miorodon. The
males (16-30 mm) are smaller than the females (23-59 mm), and there seem to be at least
some cases of sex reversal. The females evidently spawn more than once and the fecundity
is about 2 000 - 3 000 eggs.

Diaphus dwerili attains sexual maturity at a length of about 52 mm in the Caribbean,
and about 44 mm in the Gulf of Guinea. The maximum size is about 86 mm (Nafpaktitis et al.,
1977) . Studies of the otolith growth zones suggest that the species may have a one year
life cycle (Gjrfsaeter and Blindheim, 1978).

For the life history of Agyropeleous hemigymus see Section 11 (Mediterranean) .

- 53 -

8. WESTERN CENTRAL ATLANTIC

The northern Sargasso Sea is one of the most intensively studied areas in the world as
far as mesopelagic fauna is concerned. For the rest of the Western Central Atlantic the
information is more sparse, and based on micronekton net samples and acoustic studies using
resonant frequencies, which are both supposed to give underestimates of fish biomass. All
data available seem, however, to confirm that the mesopelagic fish biomass in the area is low.

Abundance

The Ocean Acre Programme, designed to study the acoustical and biological characteris-
tics of the DSL in an open ocean area southeast of Bermuda (32N 64W) , has given a lot of
information on the mesopelagic fauna in the northern Sargasso Sea. The principal gear used
for biological sampling was a 10-foot IKMT fitted with a four-chambered, discrete-depth cod-
end sampler. A 6-foot IKMT and a large Engel trawl were occasionally used (Gibbs et al. 3
1971; Brown and Brooks, 1974). Acoustical measurements were taken at discrete frequencies
ranging from 1.3 to 15.5 kHz (Brooks and Brown, 1977). Brovu and Brooks (1974) give a list
of works in progress and publications based on Ocean Acre material.

IKMT samples in 50 m depth intervals down to 1 300 m indicated a biomass about 2 g/m 2 .
Day and night hauls gave similar results (Gibbs et al . 9 1971; Krueger and Bond, 1972). Only
Myctophidae and Gonostomatidae (excluding Cyelothone) are included in these values.

The final results of the Engel t-awl samples are not published, but it is obvious that
the Engel trawl caught larger fish than the IKMT while some abundant small species were vir-
tually absent in the Engel trawl samples (Brown and Brooks, 1974).

Several models have been developed to relate acoustic profiles to fish distribution.
The first models showed good agreement in form, but the sound scattering profiles predicted
from biological sampling were consistently too low. The reason was probably avoidance of
the sampling gears (Scully-Power, 1977). To compensate for this the mean length of the fish
caught in the IKMT was adjusted according to a comparison with Engel trawl samples and a
better fit was obtained (Brooks and Brown, 1977). (No compensation for avoidance has been
applied to the biomass estimate presented in Table 8.3).

Baird et al. (1974) studied a DSL in the Cariaco Trench off Venezuela using a modified
6-foot Tucker trawl and acoustic equipment working at 12, 25 and 50 kHz. Based on trawl
catches there were about 0.07 fish/m 2 . The mean length of Diaphus taaningi, which made up
about 80% of the catches, was about 41 mm corresponding to 0.6 g. The biomass, therefore,
was about 0.05 g/m 2 . Based on acoustic data (25 kHz) they estimated a density of about 1.6
fish/m 2 corresponding to nearly 1 g/m 2 . Baird and Wilson (1977) also refer to sampling at
one station in the Gulf of Mexico and one in the open Caribbean Sea. Based on their data
biomass estimates of about 0.02 and 0.04 g/m 2 can be calculated for the two areas. These
stations were both placed in areas with very low primary production.

Backus et al. (1970) and Backus and Craddock (1977) published the mean catch in ml/hr
for faunal provinces in the Atlantic, and their results from the Western Central Atlantic are
shown in Table 8.1. The study was carried out and the result converted to g/m 2 as described
in Section 5 (Northeast Atlantic). The biomass 0.02 - 0.14 g/m 2 are among the lowest they
recorded in the Atlantic Ocean.

Chapman et al. (1974) carried out acoustic studies covering a wide range of frequencies,
and some of their results from the Western Central Atlantic are summarized in Table 8.2 to-
gether with some estimates based on their data. The estimated biomass 0.2 to 0.8 g/m is low,
but of the same order as in other parts of the Atlantic Ocean.

Chapman and Marshall (1966), however, obtained densities between about 0.8 and 1.4 g/m 2
of scatterers with swimbladder size 1.8 mm in the northern Sargasso Sea.

- 54 -

Table 8.1

Abundance estimates derived from catches by
micronekton nets in the Western Central Atlantic

Author

North
Sargasso
Sea
g/m 2

South
Sargasso
Sea
g/m 2

Gulf of
Mexico

g/m 2

Caribbean
g/m 2

Lesser
Ant il lean
Province
g/m 2

Backus and Craddock, 1977

0.06

0.02

0.14

0.08

Baird and Wilson, 1977

0.02

0.04

Baird et al.> 1974

0.05

Gibbs et al., 1971

0.20

Table 8.2

Abundance estimates derived from acoustic data
presented by Chapman et al. (1975)

Area

Scattering
Strength

N/m 2

Bioraass
g/m 2

North Sargasso Sea

.28
.34

-49
-49

0.09
0.06

81
99

5.8
10.3

0.5 )
) 1.1
0.6 )

South Sargasso Sea

.11

-48

0.7

0.25

0.2

Caribbean

.17

-45

0.6

1.22

0.8

Lesser Antillean Province

-49

0.2

2.21

0.4

R is swimbladder radius (cm), W is estimated number of fish, C and $ are estimated
length (mm) and weight (B) of the resonant fish

Synthesis of abundance estimates

Although all data available for the Western Central Atlantic are obtained by methods
which obviously will underestimate the true biomass, there is little doubt that the meso-
pelagic fish fauna in the area is poor. This conclusion was reached also by Voss (1966) who
stated that the IKMT catches in the Caribbean Sea were only 1/10 of those in the Gulf of
Guinea.

~ 55 -

Although the biomass probably is lower in the southern than in the northern Sargasso
Sea, the biomass derived for the Ocean Acre is used for both (Table 8.3, Fig. 8.1), For the
other areas the highest estimates of those presented are used (as all the methods used prob-
ably underestimated the true biomass) . This gives a total stock size of about 20 million
tonnes.

Table 8.3

Abundance estimates for various parts
of the Western Central Atlantic

Area

Size of area
m 2 x 10 11

Biomass
g/m 2

Stock
tonnes x 10 6

Gulf of Mexico

0.14

1.4

Caribbean

0.14

2.3

Sargasso Sea

0.2

12.1

Lesser Antillean Province

0.1

3.6

~ 122

19.4

Depth distribution

The depth distribution of the DSLs in the Sargasso Sea and the Lesser Antillean province
was studied by Haigh (1971) using a 10 kHz echo sounder. In the northern Sargasso Sea he
found two layers, one extending down from 500 - 550 m. In the southern part of the Sargasso
Sea there were layers at 270 - 350 m, 450 - 520 m and 550 - 620 m. In the Lesser Antillean
province (sensu Backus et al.> 1970) two layers at a depth of 300 - 350 m and 500 - 550 m
were observed.

Gibbs et al. 3 (1971) and Krueger and Bond (1972) described the depth distribution of
the Myctophidae and Gonostomatidae in the Ocean Acre area. During the daytime the best
catches of gonostomatids were obtained at depths around 500 m and those of rayctophids at
700 ra. During the night the best catches were made in the upper 100 m.

In the Cariaco Trench where the deep water is anoxic a DSL was observed at approximately
250 m during daytime (Wilson, 1972) ,

Species composition

Nafpaktitis et al. (1977) reviews all genera and species of the Myctophidae found in
the area, A key to the species of myctophids found in the Bermuda Ocean Acre area and some
that are expected to be found there is also given by Gibbs et al. (1971), For the Gonosto-
matidae, Grey (1964) and Krueger and Bond (1972) should be consulted, and for Sternoptychidae,
Baird (1971).

More than 300 fish species from over 80 families inhabit the Ocean Acre area. The
Myctophidae with about 60 species are far more abundant than any other group excluding Cyolo-
thone. The Gonostomatidae are second with about 20 species (Gibbs et al. , 1971; Krueger
and Bond 1972).

- 56 -

-O 0)

o> 6

CO QJ

P 4J

JS-

4-1 CO

CO OJ)

4-> r-i
01
0) 00

O <U

o> co

M CO

CO 0)

*O CO

P CO

co ni

e*c

- 57 -

According to Gibbs et al.j (1971) the following species are numerically dominant among
the Myctophidae in the IKMT catches:

Notolychnus valdiviae
Diogeniohthys atlanticus
Lampanyctus pusillus
Ceratoscopelus warmingi
Lobianchia dofleini

Among Gonostomatidae Cyclothone spp* (mainly C. braueri) rank first, followed by:

Vinciguevria attenuata
Bonapartia pedaliota
Pollichthys mauli
Ichthyoooccus ovatus

Except for Cyclothone, all these rank after the Myctophidae li.sted (Krueger and Bond, 1972),

The most abundant species according to Backus et at. (1970) are listed in Table 8.4.
In neritic waters off Venezuela Diaphus taaningi seems to be dominant (Baird et al.> 1974),
while D. dwnerili is dominant in the Caribbean in general (Nafpaktitis et al. f 1977).

Table 8.4

Principal species in shallow (200 m) nighttime
collections taken with IKMT (from Backus et aZ.j 1970)

Species

North
Sargasso Sea

South
Sargasso Sea

Gulf of
Mexico

Caribbean

Lesser
Antillean
Province

Diogenichthys atlantieus

Cevatoscopelus warmingi

Notolyohnus valdiviae

Pollichthys mauli

Lepidophanes gaussi

Benthosema suborbitale

Diaphua dwnerili

Lepidophanes guentheri

Lampanyatua alatus

L. nobilis

L. pusillus

Life history

Diogeniohthys atlantious is a very small species. The maximum size in the Ocean Acre
collections was 22 mm* It is presumed that the species has a one-year life cycle and that
spawning takes place over a long period with a peak in early spring (Gibbs et al., 1971).

Diaphus taaningi reaches a size of about 50 mm and sexual maturity is attained at a
length of 36 - 40 mm. A one-year life cycle, with a few specimens surviving to the second
year, is suggested. The spawning season is probably restricted in time, but it is not
known exactly when it occurs. The fecundity is about 1 000 eggs per female (Baird et al. 9
1974).

Vinciguewia attenuate reaches a size of 30 - 48 mm and has a one-year life cycle.
Spawning seems to take place during spring (Krueger and Bond, 1972).

For the life history of Notolyohnus valdiviae and Ceratoscopelus wamingi, see Section 6
(northwest Atlantic), and for Cyolothone braueri and Diaphus dwnerili see Section 7 (Eastern
Central Atlantic).

- 59 -

9. SOUTHEAST ATLANTIC

Although the southeast Atlantic is one of the few places where commercial fisheries for
myctophids have been carried out, little information is available on the mesopelagic fishes
of the area. Information on the distribution of species is in preparation, but more surveys
are needed to determine the abundance and life histories of the important species.

Abundance

During 1971 Krefft (1974) occupied one series of stations along a line between 0S 20W
and 350S IQQW and another between 40"S 2QOW and 35S 17E. Fishing was carried out with a
1 600-mesh herring trawl, and the mean catch was about 40 fish per minute of trawling. Assum-
ing that the efficiency of the net was similar to that of a 100% effective trawl with mouth
area of 300 m (see Section 4.1), that the. trawling speed was 3.5 knots, that the mean weight
of the fish was 1 g, and that the hauls represent the mean of the upper 1 000 m, the catch
rates correspond to a biomass of about 1.2 g/m ? .

Kashkin (1967) presented data on the biomass of mesopelagic fishes based on catches ob-
tained by a variety of micronekton nets during various cruises. His data are reported in
g/m 3 and can be converted to g/m 2 by supposing that his hauls represent the mean of the upper
1 000 m. Eighteen hauls near the African coast gave about 3 g/nr while 19 hauls in off shore
areas gave nearly 1 g/m ? surface area.

Ahlstrom, Moser and O'Toolt (1976) studied the distribution of fish larvae off southwest
Africa between 19 and 26N. All sample -, were taken nearer to the coast than about 100 n
miles. From August 1973 to April 1974, the niyctophid larval abundance was higher than 10
larvae. /m 2 in about half the area studied. From August 1972 to March 1973 only a small frac-
tion of the area had larval densities above 10/rn 2 . Myctophid larvae formed nearly 10% of all
fish larvae caught, and of these Lampanyctndes hecttor*is formed 85%. About 93% of the larvae
of this species were taken during August-November, while 62% of the young larvae (less than
5 mm long) were caught during August, indicating a short, distinct spawning period. The
gonostomatid Maura 1 i^u^ trmelUwi and the myctophids Symbulophorus boops and Diaphus dwnerili
were also abundant in the catches (O'Toole, 1.974, 1976).

Compared to the larval abundance found in the Arabian Sea (e.g. Nellen, 1973) the values
observed off southern Africa are low. They are also lower than the abundance observed off
northwest Africa (e.g. Aboussouan and Aldebert, 1978). It is difficult to draw conclusions
about the biomass, but as the myctophids off South Africa seem to have a more restricted
spawning season than those in the other two areas, it is possible that the stock size re-
lative to that in the other two areas is even smaller than the relative larval abundance.
It is, however, not known how large a fraction of the L. hectoris stock spawns within the
area studied.

Synthesis of abundance estimates

The fisheries carried out suggest that mesopelagic fish may be abundant along the south-
ern African coast, and the larval data published also seem to indicate a higher stock density
than that found by Kashkin (1967). His data are nevertheless used for the coastal zone
(Table 9.1), as no other estimate is available.

The mean catch rates obtained by Krefft (1974) in the offshore areas were higher than
in the other parts of the Atlantic, even though he did not fish in the area occupied by the
abundant Lampanyctodee hectoria, while those reported by Kashkin (1967) were low compared to
his results from other areas. It should, however, be noted that the reliability of Kashkin's
results may vary from area to area, as several different cruises are involved. Both studies
indicate a mesopelagic fish density of about 1 g/m :> , which is used in Table 9.1. Based on
this figure, the biomass of mesopelagic fish in the southeast Atlantic is nearly 20 million
tonnes .

- 62 -

Table 10.1

Abundance estimates derived from acoustic data
presented by Chapman et al. (1975)

Area

Scattering
Strength

N/m 2

L (mm)

w (g)

Biomass
g/m ?

North of 10S

0.18

-44

0.69

1.5

1.0

0.17

-45

0.62

1.2

0.8

South Atlantic

0.19

-50

0.16

1.8

0.3

Gyre

0.14

-50

0.29

0.6

0.2

Subtropical

0.23

-50

0.11

3.2

0.4

0.13

-50

0.33

0.5

0.2

Temperate

0.31

-47

0.12

7.8

0.9

0.10

-49

0.71

0.2

0.1

0.24

-47

0.19

3.7

0.7

0.17

-49

0.25

1.2

0.3

Antarctic

0.21

-50

0.13

2.5

0.3

convergence

R is swimbladder radius (cm); N is estimated number of fish; L and W are estimated
length (mm) and weight (g) of the resonant fish. For distribution of stations see
Fig. 10.1.

To estimate the total stock of mesopclagic fish in the Southwest Atlantic, the region
can be divided into three areas, with division lines following 20S and 40S latitude. For
the southern area, the data presented by Kashkin (1967) are supposed to be the most reliable
of those available. For the remaining areas the estimates are based on Chapman et al. (1975)
It is assumed that not more than half of the mesopelagic fish present are resonant at the
frequency bands used. Therefore, the biomass causing resonance has been multiplied by a
factor of two. This procedure will certainly give a conservative estimate.

Table 10.2 shows a wOtal estimated biomass of about 40 million tonnes. This excludes
stocks which may be found in the highly productive waters just off the shelf.

Depth distribution

No data on the depth distribution of the mesopelagic fish in the area seem to be avail-
able at present.

Snecies composition

A comprehensive checklist of the mesopelagic fish in the area is given by Parin et al.
(1974). The Myctophidae will also be treated by Hulley (in prep.) and McGinnis (in press).

SUBTROPICAL
2 CONVERGENCE

Figure 10,1 Stations where Chapman e.t at.
(1975) made the acoustical measurements
referred to in Table 10.1. (Redrawn from
Chapman et al.)

90 W

ION

Fig. 10.2 Subareas of the Southwest Atlantic
used in the biomass assessnent (Table 10.2).
The. figures indicate the biomass in g/irr

80 W 70 60 50 40 30 ?0

- 64 -

Table 10.2

Abundance estimates for subareas of the Southwest Atlantic

Area

Size of Area
m'x 10 n

Biomass g/m 2

Stock
Tonnes x 10 6

North of 20S

20S - 40S

South of 40S

180

Parin et at. (1974) found Cycloihone spp. (mostly C. micvodon) to be the most abundant
fish in the area followed by Bathylagus antarcticus* Gymnosoopelus braueri, Diaphus theta
and Vinciguevria nirnbaria. Diaphus dwnerili which Krefft (pers. com.) found to be densely
concentrated off Uruguay was caught only in small numbers by Parin et al. (1974).

A more detailed description of the species composition and distribution of the species
within the area must await the contributions from Hulley (in prep.) and McGinnis (in press).

Life history

The life history of Cyclothone microdon has been described by Badcock and Merrett (1976)
and some of their results are presented in Section 7 (Eastern Central Atlantic).

A few life history data on Diaphus dwerili were given by Nafpaktitis (1968) and by
Gjrfsaeter and Blindheim (1978). These are summarized in Section 8 (Western Central Atlantic)

For the other important species found in the area, life history information is very
sparse.

- 65 -

11. MEDITERRANEAN SEA

Studies of mesopelagic fish have long a tradition in the Mediterranean. Important con-
tributions were presented as early as 1918 (Taning, 1918). The life history of the important
species is, therefore, well known compared to what is known about species in most other areas,
Quantitative data are, however, still few, and they all refer to oceanic areas.

Abundance

In 1970 sampling was conducted with a 10-foot IKMT fully lined with i-inch bar mesh.
The results have been presented as specimens /hr (Goodyear et al. 3 1972) and in Table 11.1
they are converted to g/m 2 by supposing that the filtering efficiency was 90% (Brooke et al.,
1974), that there were no fish below 1 000 m, and that the towing speed was 3 knots (not
stated). The mean length of the fish seems to have been about 20 ram (Appendix tables to
Goodyear et al.> 1972), probably corresponding to a weight of about 0,1 g. In the central
area (station 5) there is not sufficient data to estimate the biomass, but apparently it is
similar to, or slightly lower than that at station 2.

Backus and Craddock (1977) published the mean catch in ml/hr for the western and east-
ern Mediterranean Sea. Their data are based on nighttime hauls in the upper 200 m with a
10-foot IKMT. Assuming that the efficiency of the gear was about 90% (Brooke et al. 9 1973),
that the fish were evenly distributed in the upper 200 m and that there were no fish below
this depth during the night, the following results are derived:

Western Mediterranean Sea 0.5
Eastern Mediterranean Sea 0.03

As Cyclothone spp., which by far outnumbers all other species in the area (Goodyear et al. 3
1972), never enters the upper 200 m these figures seriously underestimate the true biomass.

Table 11.1

Fish abundance at the stations occupied by
Goodyear et al. (1972). (See Fig. 11.1)

Station

Fish/m 2 surface

Day

Night

5.5

3.3

9.1

5.5

3.3

2.7

slightly lower than
station 3

0.7

3.5

- 66 -

Acoustic studies covering a wide range of frequencies have been reported by Chapman
et al. (1975) and some of their results from the Mediterranean Sea are summarized in Table
11.2 along with the estimates derived from them. Both stations show a fairly large biomass
and they indicate that size groups not taken by IKMT may be of importance in the area.

Table 11.2

Abundance estimates derived from acoustic data
presented by Chapman et al. (1975)

Area

Scattering
Strength

N/m 2

Biomass g/m 2
Each size Total
group

Ligurian Sea

0,37

0.13

107

13.2

1.7 )

0.23

0.13

3.2

0.4 ) 2.4

0.12

0.78

0.3

0.3 )

Tyrrhenian Sea

0.35

0.12

102

11.2

1.3 )

0.25

0.11

4.2

0.5 ) 2.2

0.16

0.35

1.0

0.4 )

R is radius of swimbladder (cm); N is estimated number of fish;
(mm) and weight (a) of the resonant fish

L and W are length

Synthesis of abundance estimates

The information available is sparse, but it seems clear that the biomass of mesopelagic
fish in the Mediterranean Sea is low, and lower in the eastern than in the western part.

The fish densities used for the western part of the Mediterranean Sea and the Tyrrhen-
ian Sea (Table 11.3, Fig. 11.1) are those based on Chapman et al. (1975), raised slightly
to compensate in part for non-resonant fish. These data, derived from acoustic work, are
chosen as they show that size groups not commonly caught by IKMT are of importance. For
the other areas the estimates are based on Goodyear et al. (1972), as no other data are
available.

Depth distribution

Goodyear et al. (1972) give data on depth distribution of all the important species of
mesopelagic fish caught by IKMT. During the daytime they caught mesopelagic fishes between
100 and 1 000 m with peak concentrations at various levels from 375 to 1 000 m. During the
night peaks were observed at 0-100m and 400 - 800 m.

Species composition

An up-to-date list of mesopelagic fish found in the Mediterranean is given by Bureau
and Monod (1973). For identification of specimens the works listed for the Northeast
Atlantic should be consulted.

- 67 -

4-J 03

C3 f-H

Ol 03

03 03

03 -H

0) <N

03 r-i e

w 1 ^

CO r-l 00

r I
03 C

c 6

0) -H -H

r ff O

!) O

iJ QJ

C J2

Q^ M 4->

O3 Q> rt

D tM O

0) H

C< M 13

cr3 C

Q) 03 -H

(tJ O 03

VJ H CU

Q) CO D

4-) 4- &JD

H 03 -H

4J 03

/*^

u-i en <u
O X

1-1 H

OD i I
flj

D H

0)
V-i

Table 11.3

Abundance estimates for various parts of the Mediterranean Sea

For areas see Fig* 11.1

Area

Size of area
m 2 x 10 11

Biomass
g/m 2

Standing stock
tonnes x 10 6

4.6

1.4

2.4

0.7

6.8

0.5

0.3

4.6

0.1

0.05

18.4

2.45

Table 11.4

Rank of catch rates in IKMT in the Mediterranean Sea
Based on data from Goodyear et at. (1972)

Station Area

Off S.E.
Spain

W. of
Corsica

Tyrrhenian
Sea

Ionian
Sea

S. of
Crete

Cyclothone braueri

C. pygmea

Benthosem glaciate

Ceratosoopelus mderensis

Lampanyctus pusillus

Hygophw benoiti

Lobianohia dofleini

Goniohthys COOQO

- 69 -

Goodyear et al. (1972) found that Cyclothone braueri gave the highest catch rates in
all areas of the Mediterranean (Table 11.4). Among the most important myctophids were Ben-
thosema glaciale, Ceratoeoopelus maderensis and Gonichthys QOQCO.

Aboussouan (1971) collected fishes off the Provence coast using an IKMT. The Sternop-
tychidae (Argyropelecus hemigymus) made up 16%, Gonostomatidae (mostly Cyclothone braueri)
27%, and Myctophidae (most Benthosema glaoiale and Diaphus raphinesqui) only 8% of the
catches.

Dekhnik and Sinyukova (1966), studying fish larvae, found that the Gonostomatidae
(Cyclothone and Vinoiguerria) dominated, followed by the Myctophidae of which Diaphus holti
and CeratOBcopelus maderensis made up the densest concentrations.

Life history

The life histories of important mesopelagic fish species from the Mediterranean have
been studied by Taning (1918), Jespersen and TSning (1926) and Goodyear et al. (1972). Most
of the following account is based on Goodyear et al. (1972).

Cyclothone pygmea are much smaller than fish of the same species in the North Atlantic.
The maximum size seems to be about 41 mm and sexual maturation occurs at a length between
30 and 35 mm in both males and females. Spawning probably peaks in late spring and summer,
after which the adults die.

CeratoscopeluB mderensis reach a size of about 70 mm. All specimens larger than 38 mm
seem to be mature, and spawning apparently occurs in spring and summer. Probably the species
has a one-year cycle and the older fish die after spawning.

Hygophum benoiti is a small species reaching a maximum size of about 45 mm. Sexual
maturity is reached at a length of about 30 mm and a one-year life cycle is suggested.
Spawning apparently peaks in spring and summer.

Argyropelecus hemigymus seem to reach a size of 20-24 ram, the males being slightly
smaller than the females. There seems to be a restricted breeding season, and most of the
adults apparently die after spawning.

- 71 -

12. WESTERN INDIAN OCEAN

The Arabian Sea has been fairly intensively studied as far as mesopelagic fish are con-
cerned. Acoustic records, egg and larva counts and catch data from micronekton nets and com-
mercial trawls are all available. For the southern Indian Ocean much less information is
available and most data are from micronekton nets, although acoustic equipment with echo
integration and commercial trawls have been used off Mozambique and the Seychelles.

Abundance

During the Indian Ocean Expedition, R/V ANTON BRUUN collected mesopelagic fishes between
10N and 45S along 60E and between 18N and 40S along 65E. The gear used was a 10-foot
IKMT. The two most abundant families caught were the Myctophidae (Nafpaktitis and Nafpaktitis,
1969; Nafpaktitis 1978) and Gonostomatidae (Craddock and Haedrich, 1973) . Generally the catch
rates were low. Assuming that about 4x10 * m 2 /hr of water was filtered and that the efficiency of
the gear was 90% (Brooke et al. 3 1973), only a few stations yielded more than 1 fish/m 2 surface
area. Only two stations in the Arabian Sea (16 05 f and 17 46 ! N 65E) gave catch rates of
about 2,5 and 3.5 fish/m 2 , corresponding to between 1,5 and 2 g/m 2 . Three species, Hygophwri
proximwn, Bolinichthys longipes and Diaphus thiollieri^ were dominant at these stations.

During autumn 1964, R/V TE VEGA made a cruise from Mombasa to Sri Lanka (Bradbury et
at.* 1974). Mesopelagic fish was caught with a 10x10 ft Tucker trawl. The towing speed
was 1 to 1.5 knots and catches vere generally low, the best one yielding about 220 fish of
which more than half wera Cyclothone. This may correspond to about 1-2 g/m 2 . Other
stations indicated a density of about 0.5 g/m 2 or lower. It is not possible to assess any
trend in the abundance along the cruise track.

During 1975 and 1976, R/V Dr. FRIDTJOF NANSEN worked in the Arabian Sea making five sur-
veys between Mogadishu and the India-Pakistan border (Anon, 1978). On these surveys the
mesopelagic fish were studied using 38 kHz echo sounders with electronic integrators and
commercial-sized pelagic trawls (Gj^saeter, 1978a) . The abundance estimates based from echo
integration are shown in Table 12.1. The areas used are shown in Fig. 12.1. In terms of
biomass per unit surface area, the mean densities varied from about 80 - 200 g/m 2 in the
Gulf of Oman to about 20 g/m 2 off Somalia (Fig. 12.1). The highest concentrations were
usually observed just off the shelf break. Trawling confirmed that the densities of fish
were high in some areas and 26 stations gave catch rates higher than 400 kg/hr of mesopelagic
fish (Table 12.2 and Fig. 12.2). The highest catch rate obtained was 20 tonnes/hr. Catches
with a fine-meshed krill trawl indicated that the best concentrations could reach 8 fish/m 3 .

During January- June 1977 a similar study was carried out off the coast of Pakistan

(Anon, 1978a; Myrseth, in prep.). The area shown in Fig. 12.3 was covered five times, and

the abundance estimates ranged from about 3 to 13 million tonnes. Catch rates of 2 - 5
tonnes/hr were obtained in pelagic trawl hauls.

During 1977/78 the Gulfs Regional Fishery Survey and Development Project (FAO) surveyed
the Gulf of Oman using acoustic equipment (Anon 1978a) . During November 1977 and May 1978
they found 4.60 and 2,75 million tonnes of myctophids. Concentrations between 50 and 500
g/m were commonly found, and in some places the fish density was higher than 500 g/m 2 . The
concentrations were highest along the edge of the continental shelf. They also observed
that the stock size was slightly higher in the southern than in the northern half of the Gulf.

During a cruise in September 1978, they again recorded dense concentrations (50-500 g/m 2 )
alorg the edge of the continental shelf in the southern part of the Gulf of Oman, while con-
centrations were very low in the central and northern parts (Lamboeuf and Simmonds, unpub-
lished report to FAO) .

During the period August 1977 to June 1978, R/V Dr. FRIDTJOF NANSEN worked of f Mozambique.
The area from the coast to a maximum of about 100 n miles off the slope and from 10 to 27S
were covered four times. The data are still being processed, but preliminary calculations

- 72 -

30N

c /

71-52 /

-20

-15

45E

70P

Fip,. 12.1 Areas studied by R/V Dr. FRIDTJOF
NANSEN January 1975 - November 1976. The
figures indicate the highest and the lowest
nean biomass (g/m 2 ) observed in the subareas

30N

-20

Pelagic trawl

D Bottom trawl

O Pel. trawl and

krill trawl

45E

65'

-10

Fig. 12.2 Trawl stations taken by R/V Or. FRIDTJOF NANSEN
giving more than AGO kg mesopelagic fish/hr of
trawling

- 73 -

- 74 -

Table 12.1

Estimated abundance of mesopelagic fish in the areas

investigated by R/V Dr.FRIDTJOF NANSEN (in million tonnes)

Numbers in brackets are size of the areas in n mi 2 x 10 3

For areas see Fig. 12.1.

Cruise
No.

Period

Area
ABCD EFGH1
(26. 6) (27. 7) (95.0) (109.1) (30. 2) (43. 6) (89. 3) (37.1) (29. 5)

Total
(488.1)

1.2

Spring 1975

20 8 23 15 12 28 26 10 6

148

Autumn 1975

8 6 19 17 12 16 20 6 3

107

Spring 1976

13 7 23 15 5 11 31 5 3

113

Summer 1976

11 7 17 624612

Autumn 1976

15 5 20 11 3 4 20 3 3

Mean

13 7 20 13 7 13 21 5 3

102

(unpublished) show that about two million tonnes of mesopelagic fish may be present in the
area. The highest concentrations (about 10 - 30 g/m ? ) were observed just off the shelf.
The catch rates were always much lower than in the northern Arabian Sea.

During the summer and autumn of 1978, R/V Dr.FRIDTJOF NANSEN also worked off the
Seychelles and off Sri Lanka. The final results of these cruises are not available yet,
but a preliminary assessment indicates that the abundance of mesopelagic fish is very low
compared to the Arabian Sea.

Nellen (1973) reports studies of eggs and larvae in the Arabian Sea from December 1964
to April 1965. The Myctophidae were the dominant group of fish larva with the Gonostomatidae
ranking next. In the Gulf of Oman the Myctophidae amounted to 335 larva/m 2 . In the other
areas it ranged from 12 to 37 larva/m 2 . The Gonostomatidae ranged from 2 to 8 larvae/m 2
(Table 12.3). In the Gulf of Oman and off Pakistan one species, Benthosema p-berotwn, made
up the bulk of the samples. The data do not provide any information on seasonal variation.

Fursa (1969, 1973 and 1976) studied fish eggs and larvae in various parts of the Arabian
Sea using large, conical ichthyop lank ton nets fished in depth strata down to 100 m. The
abundance of larvae is reported as number of larva/100 m 3 . Supposing that the larvae were
distributed in the upper 100 m, this equals larva/m 2 surface area.

Fursa (1969) collected larva along the western shore of the Indian peninsula during the
winter (January-March) and summer (June-July) monsoon, finding 14 and 6 larva of mesopelagic
fish/m 2 surface area in the two periods, respectively. In waters of depth between 180 and
950 m the abundance was 15 and 10 larva/m 2 and in waters deeper than 1 000 m he got 26 and
19 larva/m 2 , in the winter and summer monsoon, respectively (Table 12.3).

- 75 -

Table 12.2

Trawl stations taken by R/V Lr. FRIDTJOF NANSEN
with catch rates of myctophids > 400 kg/hour

St. No.

Date

Area

Trawl

Trawl-
depth

Time

Total
catch
kg

Catch
mycto-
phids
kg

Myctophids
kg/hr

Dominant
species

1975

168

19.9

180

day

5 210

5 000

9 400

219

17.10

180

day

1 500

3 000

234

3.1J

night

300

230

400

P. ptervtwn

239

7,11

250

day

610

600

1 200

B. pterotwn

1976

281

31.1

night

410

405

810

P. ptarotum

310

22.2

280

day

2 000

6 000

P. fibu'Latum

314

26.2

270

day

250

230

460

/:'. ptepotwn

319

28.2

night

900

800

1 600

B. pterotwn

320

29.2

night

1 500

3 000

B. pterotum

325

5.3

140

day

800

780

1 560

B. pterotwn

326

5.3

night

450

440

880

D. pterotum

327

5.3

100

day

450

430

860

P. pterotum

329

6.3

night

5 000

10 000

B. pterotum

330

6.3

130

day

650

1 300

P. pierotum

352

26.3

day

300

200

400

B. pteroiwn

419

25.5

200

day

1 500

3 000

Diaphun spp.

427

3.6

130

day

10 000

20 000

B. pt&rotwn

433

10.6

100

dawn

1 000

2 000

P. pterotwn

434

10.6

175

day

600

500

1 000

B. pterotwr\

436

13.6

night

300

600

B. pterotum

448

20.6

night

1 500

1 300

2 600

P. pterotwn

449

20.6

night

800

700

1 400

Z?. pteroturri

450

20.6

night

1 500

1 300

2 600

B. pterotwn

451

20.6

night

500

400

800

B. pterotwn

469

29.8

300

day

1 000

B. pterotum

543

26.10

120

night

10 000

9 600

19 000

B. fibula-turn

P * pelagic trawl (1360 meshes); KT - krill trawl; BT - bottom trawl

- 76 -

Table 12,3

Larval studies in the Indian Ocean
The figures represent larvae/m 2

Author

Period

E. Atrica
M G

G. Aden
M G

G. Oman
M G

Pakistan
M G

India
M G

Open Ocean
M G

Fursa 1969
*

Jan. -Mar. 1967
Jun.-Jul. 1967

6 8
5 1

Fursa 1973
*

Jan. -Apr. 1969
Jul. -Sept. 1969

43 3
4

20 1
3

October 1969

4 1

Nellen 1973

Dec. -Apr. 1965

12 4

21 4

335 2

37 5

15 8

Fursa 1976
*

August 1971

2 3

Ali Khan 1976
**

November 1964
March 1967

44 2
41 9

March 1968

40 4

* Larva per 100/m 3 larva/m 2 if they are evenly distributed in upper 100 m
** Mean of stations off or near the 200 m line
M Myctophidae; G = Gonostomatidae

Fursa (1973) collected larvae off the coast of Pakistan and in the Gulf of Oman. Most
stations were in offshore waters. Except a section near the mouth, the Gulf of Oman was
only sampled during the winter monsoon, when 46 mesopelagic larvae/m 2 were recorded. Off
the coast of Pakistan 21 larvae/m 2 were caught during the winter monsoon, 3/m 2 during the
summer monsoon and 5/m 2 between the monsoons (October) (Table 12.3)

Fursa (1976) again collected larvae along the western shore of the Indian peninsula
during the summer monsoon. Most samples were taken in shallow waters and no data for dis-
tinguishing between those taken in shallow and those taken in deep waters are given. Along
the northern part of the coast (north of Bombay) about 8 larva/m 3 were caught, and along
the southern part about 3 larvae/m 2 (Table 12.3).

Off the coast of Pakistan, Ali Khan (1976) collected larvae with an Indian Ocean stand-
ard net. The sampling was carried out in the winter monsoon periods of 1964, 1967 and 1968.
The number of mesopelagic fish larvae caught at stations near or outside the 200 m depth
contour amounted to about 40 - 50 larva/m 2 during the three years. Very few larvae were
caught at the shallow stations. The data (Table 12.3) seem to suggest that there are more
larva during the winter monsoon than during the summer monsoon. The data also indicate that
the highest larval concentrations are found in the Gulf of Oman and off Pakistan.

- 77 -

As too little is known about seasonal variation in spawning, fecundity of the species
involved and the age of the larvae sampled, it is difficult to interpret the data in terms
of fish abundance. They indicate, however, that the mesopelagic fish biomass in the Gulf
of Oman and off Pakistan is higher than off northwest Africa (Section 7), while the other
parts of the Arabian Sea may have densities similar to those off northwest Africa.

Synthesis of abundance estimates

Acoustic estimates, catch rates in commercial trawls and larval sampling all show that
mesopelagic fish biomass is very high in the north Arabian Sea. Larval sampling indicates
that the abundance may be similar along the western coast of India, but the information from
that area is not as good. South of Somalia and in offshore areas the abundance is much lower.

The estimates given in Table 12.4 are based on the DR FRIDTJOF NANSEN surveys for areas
1 and 4. For area 2, larval abundance is compared with that in area 1, and based on this
ratio a stock estimate is derived. For areas 3 and 5, the estimates are based on catches
from micronekton nets, although the low values seem to be confirmed by acoustic studies in
parts of these areas

Nearly one half of the mesopelagic fish biomass recorded from the western Indian Ocean
is found in nearshore areas of the Arabian Sea (Table 12.4). It is, however, important to
note that both the results of the DR FRIDTJOF NANSEN and those of the Regional Fishery Sur-
vey and Development Project from the Gulf of Oman suggest that there may be large variations
in abundance in the north Arabian Sea. More work is needed to assess the short-term and
long-term variation in this area.

Table 12.4

Abundance estimates for various parts of the Western Indian Ocean

For areas see Fig. 12.4

Area No.

Size of area
m 2 x 10 11

Biomass
g/m 2

Stock
Tonnes x 10 G

100

0.5

3.5

184

0.5

304.5

257

Depth distribution

In the neritic areas of the Arabian Sea a very dense mesopelagic fish layer was usually
observed between about 150 and 200 m and a more diffuse one between 250 and 350 m (Fig. 12.5),
During the night most of the fish concentrated in the upper 50 m.

In offshore and southern parts of the Arabian Sea a DSL was usually observed at 250-
350 m and sometimes an additional one was found between 350 and 500m. This larva layer, or part
of it, was sometimes observed in the same depth during the night, too (Gjrfsaeter, 1978a) .

- 73 -

30 N

Figure 12.4 Subareas of the W.Indian Ocean
used in the biomass assessment (Table 12.4).
The smaller figures indicate the biomass in g/m 2

3Ot 40 50 60 7O 80

2 !P '? 2p ?2

02 04 06 06 10

Figure 12.5 Vertical migration of DSLs as observed with 38 kHz echosounder

during a diurnal station in the Gulf of Oman taken by R/V Dr. FRIDTJOF

NANSEN, March 1976: 1) schools and very dense aggregations,

2) dense recordings, 3) scattered recordings. (From Gj^saeter 1978a)

- 79 -

Further to the south, Bradbury et al. (1971) observed the main DSL from 300 - 350 m
downward. Sometimes two bands were observed, the lower one centred at or below 500 m.

Species composition

In the highly productive areas of the Arabian Sea Benthosema ptewtwi was the dominant
species, but B. fibulatw., Hygophurn proximwj Bolinichthys longip&fi and Diapkun thiollierei
were also abundant (Nafpaktitis and Nafpaktitis, 1968; Gj<zisaeter, 1978a; Nafpaktitis, 1978).

Further to the south the diversity was higher. In equatorial waters Nobolyjhnus valdiviae
seems to be important in offshore areas (Bradbury et aZ,, 1971).

In nearshore waters off Mozambique, Benthosema fibulatwn f Diaphus pernpivHlattis and
D. watasei are most frequently caught.

Life history

Aspects of the life history of the two dominant species, from the Arabian Sea, bcnthosewa
pterotw and fl. fibulatwn, were studied by Gj^saeter (1978a).

5. ptevotum grow to a maximum size of about 50 mm, but specimens larger than 45 mm are
rare. Rings in the otoliths, which are presumed to be growth zones, indicate that the max-
imum size is reached within one year. Reproduction is continuous but with maxima in March-
June and September-November (Makhdoom, In prep.). Feeding generally takes place during the
early evening and copepods and various crustacean larvae setim to be the dominant food items.

B. fibulatm reach a maximum size of about 100 ram, but specimens larger than 80 mm are
uncommon. Rings in the otoliths suggest that the species has a one-year life cycle. Sexual
maturity seems to be reached at a length of about 40 mm.

Diaphus watasei and D, aontleun are among the largest myc tophi d species known, both
reaching a size between 150 and 300 mm. Adults of both species are usually caught at or
near the bottom both day and night (Nafpaktitis, 1978; Cjisaeter, 1978 and unpubl.).

- 81 -

13. EASTERN INDIAN OCEAN

The available information on the abundance of mesopelagic fish in this area is very
sparse, especially in the subantarctic area. The distribution of biomass could therefore
only be estimated from scattered micronekton net data. The area was divided into four sub-
areas I - IV based on net catches and the distribution of primary production and zooplankton
abundance. The total biomass was estimated from converted IKMT-10 ft values for each area.
The results of some zoogeographical surveys are also available, which are useful as indicators
of species composition.

Abundance

Legand (1969) reported the seasonal variation in biomass of micronektonic animals at
ten stations along 110E between 9S and 32S, based on IKMT-5 ft samples collected at
night in the upper 200 m layer. The sampling was done every month from August 1962 to August
1963, except December. Annual mean fish biomass is reported to account for 79% of micronek-
ton biomass in dry weight, but seasonal fluctuations were reported only for the total micro-
nekton. Among his results, the following are thought important for the purposes of this
review:

1. The biomass of the micronekton showed a smaller variation both chronologically and
geographically than did the plankton.

2. Total micronekton biomass along 110E between 9S and 32S decreased from January
to March.

A calculation of fish biomass from the annual mean of the micronekton biomass at nine sta-
tions is shown in Table 13.1. For the calculation, the annual mean percentage of fish bio-
mass in the total micronekton biomass was taken to be 79%, the conversion ratio from dry
weight to wet weight as 4.3, and the volume of water filtered per standard haul as 19 290m 3 ,
all of which were measured or assumed by him. The efficiency (catchability coefficient) of
the IKMT-5 ft was assumed to be 0.8.

Table 13.1

The relative abundance of micronekton in the upper 210 m layer at night

along 110E between 9S and 32S in the southeastern Indian Ocean

The values were read from Legand (1969, Fig, 3)

930'S

12S

15S

18S

21S

24S

27S

30S

32S

Micronekton biomass
(g dry wt/haul)

9.1

8.2

6.6

5.9

5.5

4.9

7.1

6.9

5.5

6.7

Fish biomass
(g dry wt/haul)

7.2

6.5

5.2

4.7

4.3

3.9

5.6

5.5

4.3

5.3

Fish biomass
(g wet wt/haul)

31.0

28.0

22.4

20.3

18.5

16.8

24.1

23.7

18.5

22.8

Fish biomass
(g wet wt/ra 2 )

0.40

0.38

0.30

0.28

0.25

0.23

0.30

0.25

0.30

- 82 -

The abundance of mi cranek tonic fish was measured in the upper 1 000 m by a ring net
(OKI-net) along 86E latitude between 16N and 20S in February - March 1977 (Kawaguchi, un-
published). The results are shown in Fig. 13.1 together with those of Legand (1969) and the
distribution of annual primary production from FAO (1972).

Synthesis of abundance estimates

Reliable data are very few for this area, but we may safely estimate the lower limit of
the order of magnitude of biomass by the following considerations and procedures. The abund-
ance estimated from ORI-net data collected in February - March is thought not higher than
the annual mean, according to Legand f s (1969) results on seasonal fluctuation of micronek-
ton biomass. The abundance of mesopelagic fish will show a less variable geographical dis-
tribution than primary production, owing to the mobility of the fish in the mesopelagic zone,
which is itself a more stable environment than the epipelagic zone. With these features in
mind, we can derive the following patterns from Fig. 13.3. In the area of the lowest primary
production (< 100 mg C/m 2 /d), the biomass of mesopelagic fish is usually low (0.5 ~ 0.8 g
wet wt/m 2 ) . Two high values (1.5 and 1.6 g/m 2 ) were also obtained in this unproductive area
near the rather complicated contour lino of primary production, but this kind of anomaly in
the distribution of primary production and mesopelagic fish biomass seems natural, if the
strong seasonal change of the physical environment in the epipelagic zone caused by the mon-
soon in this area, and the swimming ability of the fish, are taken into account. Legand *s
results show that the annual mean biomass in the upper 200 m layer at night is fairly uniform
in the area where 100 - 250 mg C/m 2 /d primary production is observed, and higher in the area
where more than 250 mg C/m 2 /d is measured.

In consideration of the foregoing we divided the eastern Indian Ocean into four subareas
as shown in Fig. 13.2. The value 0.7 g/m ? for the area of lowest production (subarea IV)
was obtained by averaging the six ORI-net values (0.5 - 0.8 g/m 2 ). All the other ORI-net
values, 0.9 ~ 3.0 g/m were averaged to obtain the value (1.5 g/m 2 ) for subarea II. The
values in subarea T and III were assumed to be 1.2 and 0.8 times as large as that in subarea
II, All these values, based on the ORI-net, were converted to equivalent IKMT-10 ft values
by using the factor 2.6, obtained in the Western North Central Water area (see Section 14,
Northwest Pacific) .

The highest value, for subarea 1, is comparable to that of the Kuroshio area north of
its axis (transitional zone), and the lowest value, for subarea TV, to that of the central
water area of the Northwest Pacific. The estimate for each subarea is presented in Table
13.2. In total the biomass of mesopelagic fish is estimated at 92.9 x 10* tonnes on the
basis of IKMT-10 ft values.

Table 13.2

Estimated total biomass of the mesopelagic fishes
in each subarea of the eastern Indian Ocean

Subareas

Size of subarea
m 2 x 10 11

Biomass
(g/m 2 )
ORI-net IKMT-10 ft*

Stock
(tonnes x 10 G from
IKMT-10 ft values)

Subarea I
II
III
IV

34
75
122
59

1.8 4.7
1.5 3.8
1.2 3.1
0.7 1.8

16
29
38
11

Total

290

Converted from the ORI-net data

- 83 -

20 N

90 E 100

f>VVI 100-150

110

^ 150-250
l&a

II > 250

I2O \6V I4O I5O

Q/Legond (1969) )KMT-5ft 1962 . Aug -v Aug 1963

\(overy month, except Dec 1962), night, 0-PlOm., in g/m 2

Kowaguchi (unpublished) OR I- net 1977, Jan-Feb, night
0-ca lOOOm(?day stations concerted to night)
in g/rn*

PRIMARY PRODUCTION IN mg C/mVd

Fig. 13.1 Distribution of the fish micronckton biomass measured by

IKMT-5 ft and ORi-net, and its relation to the distribution
of primary productivity.

- 84 -

20N

80% 90 IOO IIO I2O I3O 140'

150

1.8 g/m 2
3 I "
3.8 -
47 "

Suh;n f.i I \'
Suh.i rc;i I 1
Sul>nr(.'.-i I I

Fig. 13,2 The four subareas and values used for the
estimation of total biomass in the eastern
Indian Ocean.

- 85 -

Depth distribution

Reliable data based on opening-closing nets or acoustic instruments are not yet avail-
able for this area. In future the vertical distributional pattern of mesopelagic fish should
be studied in relation to the. oxygen minimum layer prevailing in the mesopelagic zone of the
northern Indian Ocean.

Species composition

Based on the taxonomical and zoogeographical works by Bekker (1964), Mukhacheva (1972,
1974, 1976), Gorbunova (1972), Kawaguchi et al. (1972) and Nafpaktitis (1978) , the following
species are thought to be common in the eastern Indian Ocean. There remains much to be
learned about the quantitative species composition in this area.

Gonostomatidae: CyclolJiona alba* C. pn&i,i.*p'i7li,:u*> C. pall i da, C. "il^-podon,

C. aaalinidenGj (Sonar, term eton^tu^ ./. a*.lnnii(*wi> Vinsig
ia, Bonapartia pgdaliota, /to v/>^ hi a "^uGirvGh'a.

Myctophidae: BenthoB&na Gi4borb'ital^ 3 Diogenichthys aii.iV:ticu$ f Hijc^phwn proxi-

mwi* MycLophwn nitiduluij A/. jp-in(:>c>ir:> f^boJ )p : :or^ ciwmanni 3
Centmbranchufi nigroocel latun, A' ;*:' , 7 yi?^:.tc MlJiviae, ItimpdnyAtus
steinbecki, L. aiatus, 7. lui-H't;', L?ri Icpfainen loiigipvQ, Cer*a-
toseop<*lu

Life history

No available data.

- 87 -

14. NORTHWEST PACIFIC

This area can be divided into three subareas according to the distribution of biomass
densities, i.e. the subarctic, Kuroshio system, and Western North Pacific Central Water
areas. Most investigations have been carried out with micronekton nets and some information
is also available from larva surveys, zoogeographical surveys and accidental catches in com-
mercial fisheries. No acoustic data are available.

Abundance

Aizawa and Marumo (1967) and Murano et at. (1976) reported the vertical distribution of
zooplankton and micronekton biomass in the upper 1 000 m of the Kuroshio Current area off
central Japan. They used a 160 cm ring net (CRT-net) equipped with an opening-closing device
and towed horizontally to collect samples from discrete depth layers. Using the same type
of net towed obliquely in the upper 1 000 m layer, Kawaguchi (1973, unpubl.) investigated
tlie geographical distribution of the micronektonic fish abundance in the western north
Pacific between 20N and 45N. According to the biomass distribution, he divided the in-
vestigated area into four subareas: the subarctic area, the Kuroshio areas north and south
of its axis, and the Western North Pacific Central Water area. The estimated values obtained
from these surveys are listed in Table 14.1. The night catch data derived from the discrete
depth sampling agree well with those obtained in oblique tows in the Kuroshio area north of
its axis.

Parin ct al. (1977) estimated me.sopelagic fish biomass in the western tropical Pacific
and defined three areas according to t) e observed distribution of abundance, i.e. the equa-
torial, the Kuroshio and the central water areas. The Kuroshio area as defined by these
authors corresponds to the Kuroshio area south of its axis as described by Kawaguchi (unpubL).

Acoustic data are not available on mesopeJagic fish abundance in this area. During the
course of an investigation to find the spawning ground of the Japanese eel, Ozawa and
Tsukahara (1971), Tsukahara fit al . (1974), Kawaguchi (1.974), Ozawa (1976) and Matsui f-'t al.
(1976) reported the dominance of nicsopelagic fish larvae in the Kuroshio and central water
areas. Similar results have been obtained by Mclchikova (1969), Odate (1961) and Hattori
(1964) off Japan. But these data are not suitable, for estimating absolute biomass.

S yn the s is of a bund anc e e s t ima t e s

Considerable differences in sampling efficiency have been observed between day and night
tows and between ORl-net and TKMT tows. Therefore, we adopt here the average biomass esti-
mated from night catches by both nets for calculating a lower limit of the true biomass. In
Table 14.2 IKMT values in the Oyashio area and the Kuroshio area north of its axis were cal-
culated from the ORT-net data by using the conversion ratio 2.6 from the central water area
where data for both net types are available. The observed conversion ratio is much higher
in the. Kuroshio area than in the central water (3.9 vs. 2.6), but the IKMT data of Parin et
al. (1977) for the Kuroshio area south of its axis (on which the higher ratio is based) is
possibly higher than the average, since their sampling stations were located in the compara-
tively productive area near oceanic islands such as the Ryukyu and Bonin Islands. Therefore,
we adopted here the lower ratio from the central water to avoid an over-estimation.

For the calculation of total biomass, the northwest Pacific area was divided into three
subareas based on observed biomass density: these areas generally conform to the three major
water masses; subarctic waters, waters of the Kuroshio system and Western North Pacific
Central Water, as defined by Sverdrup et al * (1946), although their boundaries are slightly
shifted in accordance with the abundance of mesopelagic fish, zooplankton or primary produc-
tion, as shown in Fig. 14.1. To calculate the total biomass in each subarea, we applied the
value of the Oyashio area to the subarctic area (Table 14.3). The value for the waters of
the Kuroshio system was obtained by averaging the values on the two sides of the Kuroshio
axis.

- 88 -

-30

l6o E NO* I2O 130 140 150 ISO

E-V:V:/.- :\ 1 .3 g/nv- W. N P central water
E%%^5.2 " - Kuroshio system area
Iili|i|6.5 " - Subarctic area

170

ieo

Fip,. 14.1 The three subareas and values adopted for the
total biomass estimation of mesopelap,ic fishes
in the Northwest Pacific.

- 89 -

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- 90 -

Table 14.2

The average night catch by OKI-net and 1KMT-10 ft in the

major three subareas of the Northwest Pacific.
Data in parentheses were calculated from the OKI-net data
by the conversion ratio 2.6

Areas

ORI-net

Method
IKMT-10 ft

Conversion
Ratio

Subarctic area

2.5 g/m 2

(6.5) g/m 2

(2.6)

Kuroshio system area

north of its axis

2.1

(5.5)

(2.6)

south of its axis

1.2

4.7

3.9

average

1.7

5.2

4.2

Western North Pacific
Central Water

0.5

1.3

2.6

Table 14.3

Estimated total biomass of mesopelagic fish in the
Northwest Pacific, based on ORI-net and IKMT data

Subareas

Size of area
m 2 x 10 11

Biomass
g/m 2
ORI-net IKMT

Stock
tonnes x 10 fi
ORI-net IKMT

Subarctic area

2.5 6.5

6 16

Kuroshio system area

1.7 5.2

7 21

Western North
Pacific Central Water

0.5 1.3

5 12

Total

18 49

- 91 -

The results, presented in Table 14.3, show the estimated total biomass in the northwest
Pacific to be 18 million tonnes according to OKI-net data and 49 million tonnes according to
IKMT^data, The two different results are presented here to show the difference in catching
ability of the two micronekton nets towed in different ways.

The southern Japan Sea is inhabited by only one mesopelagic species, Maurolimsmuelleri
which has been known to form dense schools near the edge of the insular slopes (Okiyama, 1971).
Assuming its total range to be the long narrow area 10 km x 600 km along the edge of shelves,
and its density to be the same (5.2 g/m 2 ) as in the Kuroshio system, the total biomass is
3.1 x 10 tonnes. This value seems rather low but its availability as a potential resource
should be stressed here.

The Okhotsk Sea is inhabited by Stenobranehius nannoehir and Lampany&tuc joydani and
the Bering Sea also by several species (Kulikova, 1954; Bekker, 1967). But these areas,
which are closed by ice in winter, were excluded from the present estimation because of the
lack of any abundance data.

Depth distribution

Aizawa and Marumo (1967) reported a bimodal vertical distribution of mesopelagic bio-
mass at night with the maximum biomass in the upper 200 m layer and another peak in the
800 - 1 000 m layer. During the day, the peak was observed in the 400 - 600 m layer where
the minimum biomass was observed at night. They also pointed out the possible recruitment
of biomass from the deeper layer into the 800 - 1 000 m layer at night. In the. same area,
Murano et al. (1976) observed the peak biomass in the upper 100 m layer at night and in the
500 - 700 m layer during the day, although the bimodal vertical distribution was not so
clear as observed by Aizawa and Marumo (1967).

Parin et al. (1977) found a similar bimodal distribution in the Kuroshio, the Southeast
Asian Seas, the equatorial current and the Western North Pacific Central Water area. This
bimodality may result from the diurnal recruitment of biomass into the upper 100 m layer
mainly from the 300 - 600 m layer while the considerable biomass of non-migratory species
remains below 500 m.

gracile> one of the dominant species of mesopelagic fish in the Kuroshio area,

is known to occur at maximum density in the 300 - 700 m layer both day and night, with a

small amount of its biomass migrating up to the shallower layers, probably following some
vertically migrating prey.

The diel migration of M. muelleri has been studied by acoustics on the insular shelves
and slopes of the southern Japan Sea (Kawaguchi, 1967). It is known to migrate to the upper
100 m at night and spend the day near the bottom.

Species composition

Zoogeographically, the Northwest Pacific can be divided into four subareas: the sub-
arctic area, the subtropical area, the transitional area between the above two and the slope
water area along Japan south of 40N.

In the subarctic area, mainly north of 40 N with its boundary shifted northward in the
offshore area, the most abundant species are the myctophids Tarletonl>eania crenularis, Sym-
bolophorus calif orniensis^ Diaphus theta 9 Stenobrachius nannochir ., 5. leuoopsarus and Lam-
panyctus jordani, (Kulikova, 1960, 1961; Bekker, 1964, 1967). The biomass of the surface
migrating myctophid species, S. calif OTniensis and T. crenularisj which are abundantly col-
lee ted in night surface tows from the side of the ship tends to be underestimated in the
data obtained by oblique tows from the stern, mainly owing to disturbance by the ship. The
nonmigratory gonostomatid species Cyclothone atvaria, C. pallida* C. pseudopallida, C. alba
and Gonostoma gracile also commonly occur in this area, although the Bering Sea is inhabited
by only one species, C. atraria (Mukhacheva, 1954, 1964, 1972, 1974). It should be noted
that the subarctic area is not inhabited by vertically migrating gonostomatid species.

- 92 -

In the transitional area around 40 N, species of both subarctic and subtropical regions
occur, and the endemic species Protomyotophum crookeri and Notoseopelus japonieus are also
abundant. N. japonicus , a large myctophid reaching more than 15 cm, is rarely collected in
micronekton nets but its high abundance in this area may be inferred from its frequent occur-
rence in the stomachs of fur seals wintering in the transitional area off Japan (Japan Fish-
eries Agency, 1967).

In the subtropical area, including the Kuroshio system and the central water areas, the
data on surface-migrating myctophids have been collected by night surface tows of larva nets
(Sarenas, 1954; Odate, 1961; Hattori, 1964; Bekker, 1964; Kawaguchi and Aioi, 1972; Kawaguchi
et al. y 1972). The principal surface-migrating species are listed in Table 21.3. Concern-
ing the midwater migrants and non-migrants, the data have been obtained mainly by oblique or
horizontal tows of IKMT and ring trawls. The following species usually dominate the catch
in number or wet weight.

Myctophidae: Diogeniohthys atlantiaus, Benthosema suborbitale , Diaphus kuposhio^
D. sohmidtiy Notoocopelus respelendens.

Gonostomatidae: Vinciguerria nimbaria, Gonostoma gracile, Cyolothone atraria*

0. alba> C. pseudopallida^ C. pallida.

The slope water area, south of 40N off Japan, is inhabited by several species of
medium-sized and large myctophids (Kosaka et al. ^ 1967; Kawaguchi, 1976; Kawaguchi and
Shimizu, 1978), The larger species, reaching 120 mm or more, are often taken in commercial
midwater trawls fishing sergestid shrimp in Suruga Bay off central Japan and in bottom trawls
in commercial fisheries carried out on the insular slopes off central and southern Japan.
But they are rarely collected by micronekton nets, which they avoid. The fishermen locally
consume some of these slope water species, Diaphus wa-t,asei, D. suborbitalis and D. sagamien-
siSf although they are always caught accidentally and not landed for sale. In view of their
commercial potential, the main slope water species are listed below (Kawaguchi, 1976):

Larger myctophid species:

Diaphus Datasei* D. sagamienviSj D. gigas> D. adenomus.
Medium and smaller myctophid species:

Benthos&na pterotum^ Diaphus suborbitalis^ D. garmani.
Life history

Myctophum affine (- M. nitidulwn) is one of the dominant surface migrating myctophids
off Japan (30~ 40N) , and its life history has been studied by Ogawa (1961), Odate and
Ogawa (1961) and Odate (1966). The species is known to spawn during spring and summer. The
young reach ca. 45 mm standard length one year after hatching, ca. 60 mm after two years
and ca. 70 mm after three years. The spawning group is reported to be three years or older,
but the total lifespan is unknown. The fecundity was counted at about 8 000. Mature adults
seem to stop the diel vertical migration to surface layer, as their frequency of occurrence
in night surface tows is remarkably low during the spawning season.

Diaphus suborbitalis > which is the abundant slope water species in Suruga Bay off
central Japan, spawns mainly in summer. The juveniles, 15 - 20 mm in autumn, rapidly grow
to about 50 mm SL by the next summer, and after that the growth rate gradually decreases.
The following von Bertalanffy growth equation agrees well with the observed growth:

It * 60.03 {1 - exp [- 0.11 (t + 2.57)]}
where It = size at age t
t age in months

- 93

The maximum size observed was 67 mm SL at the age of 2.5 years (Go et al., 1977b). Knowledge
of spawning behaviour is still insufficient for this species.

Gonostom gracile, one of the most abundant gonostomatids in theKuroshio area (Kawaguchi,
1973), spawns mainly in winter and spring. The young reach maturity as males about one year
after hatching at a size of 50 - 60 mm SL, and then breed with two-year old or older females.
After breeding, mainly in summer, the males change sex to female and spawn as such in the
next breeding season. The maximum size observed is 120 mm SL. The female may survive for
at least two years (Kawaguchi and Marumo, 1967).

Maurolicus melleri is the only micronektonic fish species from the Pacific which has
succeeded in establishing itself in the Japan Sea, and it predominates there. Its spawning
is intense in spring, the average fecundity being about 300, The eggs float in the 10-75 m
layer, and the larvae have been observed in the 50 - 70 m layer, although no sampling was
done below 75 m (Nishimura, 1959; Okiyama, 1971), Annual fluctuations in the abundance of
M. muellen eggpand larvae are much smaller than those of three species of clupeoids in the
area, namely Sardinops melanosticta, Etwmeus microps and Engmulis japonica (Okiyama, 1971),

- 95 -

15. NORTHEAST PACIFIC

This area can be divided into two subareas: the subarctic and transitional areas.
These areas correspond to the eastward extension of the subarctic area and the Kuroshio area
north of its axis from the adjacent Northwest Pacific. Extensive surveys have been done with
micronekton nets in the transitional region, and there are also data from acoustic and larva
surveys, but the latter are insufficient for abundance estimation.

Abundance

Based on IKMT-6 ft sampling in the upper 225 m layer at night, Aron (1962) reported the
relative abundance of several dominant species of mesopelagic fish in a wide area of the
Northeast Pacific, including the southern Bering Sea. The results can be summarized as
follows: 1) Biomass density in the southern Boring Sea was at least as groat as in the sub-
arctic area south of the Aleutian Islands. 2) A clear boundary was observed corresponding
roughly to the 45N line in the offshore area. The quanti tative change was by a factor of 10
and the qualitative change was such that there was very little species overlap between the
two sides of the boundary, at least among the animals identified to species. Considering
these results together with similar patterns in the distribution of zooplankton and primary
production (FAO, 1972), we can divide the area into the two subareas mentioned above, namely
the subarctic and transitional areas (Table 15.1).

Table 15.1

Total estimated biomass based on
micronekton net catches in the. Northeast Pacific

Subareas

Size of area
x 10 11 m 2

Biomas.s density
(IKMT-6ft)
g/m 2

Stock
10 G tonnes

Subarctic area

4.5

Transitional area
Total

3.6

Pearcy and Laurs (1966) reported the biomass of mesopelagic fish based on 152 collec-
tions made by IKMT-6 ft in the transitional area off Oregon (44 39'N, 125 15 ! W), almost at
the boundary of the two subareas defined above. The average number and wet weight per m 2 in
the - 100 m layer were:

catch in number (ind./m 2 )
catch in wet weight (g/m')
wet weight per individual (g)

Day

1.1
2.4
2.2

1.5
3.6
2.4

Furthermore, the night biomass in the 150 - 500 m and 500 - 1 000 m layers was measured as
ca, 4g/10 3 and ca. 1.4g/10 3 m 3 , respectively (Pearcy and Laurs, 1966, Fig. 3). Assuming that
the biomass was distributed evenly in the 150 - 500 m layer, these values imply a density of
1.9 g/m 2 in the 200 - 1 000 m layer.

- 96 -

Pearcy (1976) studied the seasonal change in biomass in the uppermost 200 m layer at
night at six stations located 28, 46, 84, 120, 157 and 300 km off Oregon along ca. 44 39'N
latitude, collecting 243 samples with IKMT-10 ft. A seasonal inshore-offshore shift in bio-
mass was observed and related to seasonal changes in upwelling in the area. But on the
average, biomass in the area more than 84 km offshore was ca. 3.1 g dry weight (read from
his Fig. 4) per 10 m 2 in the - 200 ra column. This value can be converted into a wet weight
of 1.4 (1.2 - 1.8) g/m 2 in the - 200 m column by the conversion ratio 0.23 0.06 which
was measured for the fish in the area surveyed.

Acoustic surveys have been restricted to the qualitative features of the DSL and its
composition (Taylor, 1968; Pearcy and Mesecar, 1971), and are insufficient for abundance
estimates. But the swimbladder morphology of some mesopelagic fishes was studied in relation to
buoyancy by Butler and Pearcy (1972), and in relation to sound scattering by Capen (1967),
and their findings show that these species will contribute greatly to acoustic abundance in
the region. Among their results, it should be noted that Stenobraohius leucopsaruSj the
most abundant species in this area, the juveniles (less than 30 mm in standard length) have
a gas-filled swimbladder, but that of adults is regressed and surrounded by fatty tissue.
In this context, Pearcy and Mesecar (1971) pointed out in their study on seasonal variation
of biomass that the decrease in biomass offshore was correlated with an increased recruit-
ment of small 5. leucopsarus with gas-filled bladders. The sound scattering strength off-
shore, therefore, may be higher during winter than summer, even though the total biomass is
lower in winter. In the subarctic area of the Northeast Pacific no quantitative data on
biomass are available, and it was assumed for calculations in this paper that relative den-
sities were similar to those in the Northwest Pacific, as explained in the next section.

Synthesis of abundance estimates

Clarke (1973), Atsatt and Seapy (1974), and Pearcy et al. (1977) reported no significant
day-night differences in IKMT catches, but Pearcy and Laurs (1966) did.

In this paper night catch data are adopted, considering the possibility of enhanced
visual avoidance of the trawl during daylight hours. The value reported by Pearcy and Laurs
(1966), 3.6 g/m 2 in the 0-1 000 m layer, is based on 90 samples from various seasons and
thought to represent a reliable average measure of density by IKMT in the transitional waters
off Oregon. Another average value by Pearcy (1976), 1.35 g/m 2 in the upper 200 m (read from
his Fig. 3) is based on 150 samples collected during the study on seasonal off shore-nearshore
variation of biomass off Oregon. This may also be reliable as an average value for the area.
If the 1.9 g/m 2 in the 200 - 1 000 m layer at night derived above is added to this value,
the biomass is estimated as 3.3 g/m 2 in the 0-1 000 m layer, which agrees well with the
3.6 g/m 2 measured by Pearcy and Laurs (1966).

According to Patullo and Lorz (unpubl. indirectly cited from Pearcy, 1978), the ocean
off Oregon is a transitional region. The water at 200 - 1 000 m depth consists of modified
subarctic water, varying from about 55% to 78% subarctic from 85 to 120 km off the coast.
Therefore, the density estimated above (3.6 g/m 2 ) was adopted for calculating the total
abundance of mesopelagics in the transitional subarea.

There is no measured value of density in the subarctic area and it was, therefore,
calculated from the value for the transitional area by the conversion ratio 1.25. This
ratio was obtained from the neighbouring Northwest Pacific, where the conversion ratio can
be derived from the ORI-net catches in both areas.

With mean densities of 4.5 g/m 2 in subarctic waters and 3.6 g/m 2 in the transitional
area, the total biomass in the Northeast Pacific was estimated at 27 x 10 6 tonnes as shown
in Table 15.2.

- 97 -

Table 15.2

Average catches of mesopelagic fishes (No. per 10 5 m 3 ) in
daytime opening-closing IKMT-MPS net tows in the upper 500 m
off Oregon. From Pearcy (1977)

Species

6911

7011

Cruise
7107

(Year
7206

and Month)
7209 7211

7302

7306

Stenobrachius leucopsavus

161

158.

68.7

66.7

9.6

32.9

8.1

Diaphus theta

78.

12.8

23J

8.2

25.2

11.3

Tarletoribeania cvenularis

103.

17.9

7.8

2.3

7.0

3.5

Taotostoma macropus

35.9

13.6

8.8

'4.0

2.9

Protomyctophum thompsoni

21.

12.4

7.2

12.1

6.3

1.7

Lampanyctus ritteri

12.

1.7

6.2

2.7

4.3

0.8

Cyelothone atraria

14.9

0.3

0.4

2.7

2.0

ProtomyGtophum erockevi

0.4

1.2

0.4

Bathylagus pacificus

4.3

0.3

Lampanyetus regalis

1.7

0.3

Depth distribution

Pearcy and Laurs (1966) reported the vertical distribution and migration of mesopelagic
fishes off Oregon, based on samples collected in the - 150 m, 150 - 500 m and 500 - 1000 m
layers during daylight and darkness. The biomass was larger during the day than the night
at intermediate depths of 150 - 500 m, whereas the opposite was true in the upper 150 m
(Fig. 15.2). No day-night difference was observed in biomass in the 500 - 1 000 m layer,
where catches were uniformly low.

Pearcy et al. (1977) sampled micronekton from twelve contiguous depth strata in the
upper 1 000 m by IKMT-10 ft equipped with an opening-closing codend. No seasonal change was
noted in the pattern of vertical migration and average depth distribution of seven common
species, but significant vertical migration was observed in five of the seven common species.
The range of diel migration was generally 200 - 400 m. Common migratory species usually
occupied depths of - 50 m at night and 300 - 500 m by day. The population of the most
common species, Stenobvaohius leueopaarus, was composed of migratory and non-migratory in-
dividuals, and showed two nocturnal peak abundances: at 400 - 600 m and in the upper 100m.
The two groups were similar in size frequency distribution.

Pearcy and Mesecar (1971) and Pearcy (1977) worked on the relationship between sound
scattering layers and the vertical distribution of oceanic animals off Oregon. 6'. leucop-
sarus, together with two species of crustaceans Euphausia pavifica and S^rgcGtes oimilis,
was caught most often and in the largest numbers from scattering layers, especially in the
upper 100 m at night. Small S. leucopsaruG , having gas-filled swimbladders, were caught in
all tows that sampled the DSL (350 - 420 m) during day or night. But the authors stressed
the difficulties of predicting the composition of DSLs without more data on scattering
properties, abundance and distribution of oceanic species, since close agreement has usually
not been found between volume scattering and midwater trawl catches.

- 98 -

7ON

4.5 g/m 2 Subarctic area
3,5 11 Transitional area

Fit;. 15.1 The two subareas and values adopted
for the total biomass estimation in
the Northeast Pacific.

I70W (60 150 140 130 120

DAY NIGHT

FISH NUMBER/1000 m 3
5432 IO '2345

150m-

FISH WEIGHT IN g/I.OOOm 3

l,000m j

Fig. 15.2 Day-night difference in the vertical distribution
of micronektonic fish biomass in the transitional
area of the Northeast Pacific (from Pearcy and Laurs,
1966).

- 99 -

Species composition

Pearcy (1977) listed the average daytime catches of mesopelagic fish species in the
upper 500 m during the years 1969 - 1973 off Oregon. Table 15.2 from his report shows the
average abundance of common mesopelagic fishes in the subarctic and transitional, areas, and
these figures supported by the results of other investigations (Aron, 1962; Pearcy, 1964;
Pearcy and Laurs, 1966, etc.). S benobvaahiuo leuaopoaruB usually ranks first in abundance.
Though their ranks varied, Diaphus theta, Tarletonbeania crenularis and Taatostorna macropua
were common and one of them usually ranked second in abundance. Protomyctopkuni crockeri is
a transitional water species and less abundant in subarctic, waters. Other common species
which are not listed in Table 15,2 include Cemtosoopelus tovnsendi, Cycdothone signala,
C. pallida and C. poeudopalHda, all of which have their distributional centres in the sub-
tropical region to the south.

Life history

The life history of Stenobraehius iGUffopsarua was studied by Smoker and Pearcy (1970).
The species spawns from December to March off Oregon, its early growth is approximately
linear with the length of yearling fish averaging 23 mm, of 2-year-olds 41 mm, and of 3-year-
olds 59 mm. The inflexion in growth occurs at about 4 years of age. A linear regression
of length (mm) on age (months) yields:

1 * 20.78 + i .59 t (r = 0.98)

r = Pearsons correlation coefficient

t - age in months

L - body length at age t:

Based on the otolith analysis of age-groups, the following von Bertalanffy growth equation
was also obtained.

T* t = 84,96 - 79.32 e ~' 34t

These growth and reproduction patterns arc similar to those reported for the same species
off Monterey Bay, California. The maximum life span by otolith analysis is 8 years, but
for the most part reliable readings were limited to fish 5 years old or younger.

- 101 -

16, WESTERN CENTRAL PACIFIC

This area can be divided into three subareas according to the distribution of relative
abundance, i.e. the Equatorial Current system area, the Western North and South Pacific
Central Water area and the Southeast Asian Seas. Biomass has been measured only with micro-
nekton nets. Acoustic surveys are needed for the slope water species in the Southeast Asian
Seas. More taxonomic work is essential for egg and larva surveys, since the Western Central
Pacific is one of the richest areas in species diversity of myctophid fishes. At present,
identification of most eggs and larvae is impossible due to a lack of studies on their
morphology.

Abundance

Gramperin and Rivaton (1966) reported the species composition and relative abundance of
mesopelagic fish taken in IKMT-5 ft sampling in the upper 300 m at night along the equator
between 160E and 100W. They recognized four zones (A - D) based on relative abundance and
faunal changes in the group of migratory species, and pointed out the importance of the vari-
able depth of the Cromwell Current along the equator in defining these zones. The oceanic
equatorial region of the Western Central Pacific, mostly occupied by zone D in their defini-
tion, showed a relatively lower abundance (in numbers caught per tow) than the neighbouring
zone C to the east (168W - 145W) .

Parin et at. (1977) measured the total biomass of mesopelagic fishes by IKMT-10 ft sam-
pling down to 1 000 - 1 500 m at 30 stations in the subtropical and tropical western Pacific.
Of the 22 values obtained in the Western Central Pacific, 10 are from the Southeast Asian
Seas, 7 from the oceanic equatorial water area and 5 from the central water area.

Their values can be converted to g/m 2 in the upper 1 000 m layer by reference to their
Figs. 41 and 42, with the results shown below:

Average Range No .

g/m samples

The Philippine and Indonesian area

in the Southeast Asian Seas 4.2 0.6-7.7 10

The Equatorial Current area 2.6 1.5-4.7 7

The Western North Pacific Central

Water area 1.0 0.5 - 1.3 5

ORI-net sampling was done in the - ca. 1 000 m layer at 4 stations scattered in the
South China Sea in March 1.977 (Kawaguchi, unpublished data). The values were similar from
station to station, ranging from 1.4 to 2.1 g/m 2 with an average of 1.8 g/m ? . Based on the
conversion ratio 2.6 obtained in the Northwest Pacific (Table 14.4), the averaged ORI-net
value 1.80 g/m 2 can be converted to 4.7 g/m 2 , the equivalent TKMT-10 ft value. This figure
(4.7 g/m 2 ) in the South China Sea agrees well with the 4.2 g/m 2 measured by Parin et at.
(1977) in Philippine and Indonesian waters.

Synthesis of abundance estimates

The Western Central Pacific can be divided into three subareas on the basis of the dis-
tribution of abundance (Fig. 16.1). These are the Western North or South Pacific Central
Water area, the Equatorial Current system area and the Southeast Asian Seas. The Southeast
Asian Seas comprise, the South China, Sulu, Celebes, Banda and Java Seas and their adjacent
waters. Although Parin et al. (1977) considered the Banda and Java Seas as falling within
the equatorial area, the density of mesopelagic fishes throughout the Southeast Asian Seas
is clearly higher than in the more oceanic Equatorial Current system area (Fig. 42 of Parin

- 102 -

2ON

-y.>| |.o g/m 2 Central Water areas

Equatorial Current System Area

Southeast Asian Seas
(aOO-3000m. depth area)

ioot

ly. 16,1 The three suharons and values adopted 1 or the cstLnation of ^

total raic-ronoktonie fish hiomass in the Western Central Pacific,

- 103 -

et a., ^77), and these two areas were, therefore, distinguished in the present review.
The distributional patterns of primary production and zooplankton abundance also recommend
separate ^ treatment of these two areas. Primary production and zooplankton abundance in the
South China Sea have been reported to be relatively low compared with other parts of the
Southeast Asian Seas, but the density of mesopelagic fish there, measured in March 1977 by
Kawaguchi (unpublished), showed no difference from other areas. The South China Sea was
therefore not dealt with separately.

For calculating total biomass, the area of the Southeast Asian Seas having depths of
200 - 3 000 m was taken from Moiseev (1971). Estimated densities came mainly from the IKMT-
10 ft data of Parin et al. (1977) as shown in Table 16.1. Mean density in the Western South
Pacific Central Water was assumed equal to that in the Western North Pacific Central Water
area (1.0 g/m ? ). This value is slightly lower than the. 1,3 g/m 2 in the central water area
of the Northwest Pacific, but this north-south density gradient seems reasonable in light of
the distribution cf biomass from the subarctic to the Equatorial Current area. The density
in the Southeast Asian Seas is estimated at the same level as in the Kuroshio system area.

The estimated biomass in each subarea is shown in Table 16. 1. The estimated total bio-
mass in the Central Western Pacific is 52 x 10 tonnes from the JKMT-10 ft data. But it
should be noted that the slope water (200 - 1 000 m) of the Southeast Asian Seas is known to
be inhabited by the larger myctophid species, especially of the genus Diaphus^ and their
biomass is not included in the estimate owing to their net avoidance (see species composi-
tion) .

Table 16.1

Estimated total biomass of mesopelagic fish in
each subarea of the Western Central Pacific

Subareas

Size of area
m 2 x 10 n

Density
8/m 2
IKMT-lOft

Stock
tonnes x 10 6

(range)

Southeast Asian Seas

4.5 (0.6-7.7)

Equatorial Current
System area

2.6

Western North and South
Pacific Central Water area

136

1.0 (0.5-1.3)

Total

- 104 -

Depth distribution

Based on more than 1 500 samples collected by IKMT-10 ft, Legand et al. (1972) reported
the vertical distributional pattern of micronektonic fishes in the equatorial and tropical
regions of the central and western South Pacific, and classified them into two groups, deep
non-migrants and midwater migrants. The non-migrants include three species of Cyolothone -
C. alba, C. aoolinidens and C. pallida - and the hatchet fish, Stemoptyx diaphana. Their
day and night ranges are between 500 and 800 m. The migrants, mainly myctophids, share their
daytime range with non-migrants, but at night rise to the upper layers and even to the sur-
face. The range of vertical migration varies from 200 - 700 m. Usually only a part of the
population was observed to ascend at night while the remainder stayed at the usual daytime
depth.

Parin et al. (1977) sampled five discrete layers down to 1 500 m in the western tropical
Pacific by IKMT-10 ft, and observed a bimodal vertical distribution of biomass at night, re-
sulting from the presence of both migratory and non-migratory species or populations. It is
remarkable that the bioraass in the 1 000 - 1 500 m layer was comparable to that in the upper
200 m at night, and that more than 70% of the biomass was composed of families other than
the Gonostomatidae and Myctophidae.

Species composition

According to Legand et al. (1972) and Parin et al. (1977), the following species are
supposed to be abundant in the Western Central Pacific:

Gonostomatidae: Vinoiguerria nimbaria, Gonostoma elongatm, G. atlantizwn,

Cyolothone alba> C. pseudopallida* C. pallida> C. acclinidens.

Myctophidae: Lobianohia gemellari* Diaphus fulgens, D. 8plendidus> D. jenseni,

D. mollis, D. parri, D. regani, D. malayanus* Lampanyctus niger>
L. tenuiformio, L. nobilis^ L. alatus, Bolinichthys longipes*
CeratoBQOpelus wamingi.

Sternoptychidae: Sternoptyx diaphana.
Chauliodontidae: ChauUodus sloani.

Due to the limitation of sampling techniques and areas, the abundance of surface-
migrating species and slope water species is considerably underestimated in the above works.
Therefore the following species should be added, the additional migrants mainly based on
Sarenas (1954), Bekker (1964, 1965, 1967) and Kawaguchi et al. (1972) and the additional
slope water species on Fowler (1934) and Kawaguchi and Shimizu (1978).

Surface migrants: Centrobranohus andrae^ C. choerooephalus 3 Hygophw proximw*

H. reinhardti > Myctophwn spinosw> M. obtusirostrwn> M. asperw*
M. aurolaternatm,

Slope water species: Diaphus watasei, D. burtoni 3 D. chrysolynchuSj D. suborbitalis .,

D. wribrooulus.

Life history

No data are available at present.

- 105 -

17. EASTERN CENTRAL PACIFIC

This area includes various water masses and zoogeographical areas. The available in-
formation on bioraass density is uneven in both quantity and quality from area to area. Five
subareas are defined here according to mesopelagic fish abundance, water mass, and faunal
zone. All estimates are based on measured or assumed IKMT-10 ft values.

Abundance

Blackburn (1968) studied the distribution of micronektonic fish biomass (ml/10m 3 ) with
a 1.5 m square net in the upper 90 - 95 m layer in the subtropical and tropical eastern
Pacific (30N to 30<>S, east of 120W) . The distribution of biomass showed a pattern similar
to that of primary production. Fish densities ranged from 3.1 to 81 ml/10 3 m 3 in the areas
where primary production greater than 250 rag C/m 2 /day has been observed, but were only 0.3 -
3.0 ml/10 3 m 3 in the more offshore area where primary production is 100 - 250 mg C/ra 2 /day.
Assuming that the settling volume (ml) is nearly equal to grams wet weight, the above values
can be converted to the biomass in the upper 100 m as follows:

productive area (> 250 mg C/m 2 /day) 0.3 - 8.1 g/m 2
offshore area (100 - 250 mg C/m 2 /day) 0.03 - 0.3 g/m 2

The mean values are not reported ^or these areas, but can be seen from Blackburn's

figure to be about 2-3 g/m ? a ->d 0.1 - 0.3 g/m 2 respectively in the - 100 m layer at

night. This indicates that densities in the productive area are equal to or greater than

those in the upper - 150 m at night in transitional waters of the Northeast Pacific (see
Pearcy and Laurs, 1968).

Blackburn et al. (1970) analysed seasonal and areal changes in abundance in three large
areas of the eastern tropical Pacific, as shown by 248 night samples collected in the upper
200 m layer with a 1.5 m square net during the EASTROPAC expedition, 1967 - 1968. In the
western area studied (16N to 320 f S, 100O30'W to 12130 f W), sampling was done in seven suc-
cessive periods of approximately two months each. The seasonal change in f ish-cephalopod
micronekton abundance (ml/10 3 m 3 ) was small, with maximum /minimum ratio less than 2. The
maximum and minimum occurred in October-November and April-May, respectively.

Sampling was also done in two different seasons, February-March and August-September,
in two neighbouring areas to the east and south. In the eastern area (1140'N to 1500 ! S,
7945'W to 9545'W), no significant seasonal change was observed, but significant latitudi-
nal changes were. No seasonal or areal variation was observed in the more offshore southern
area (south of 3O20'S and from lO^O'W to 12130'W).

The distributional pattern of the abundance of the f ish-cephalopod micronekton agrees
with that of fish only reported by Blackburn (1968):

productive area (> 250 mg C/m 2 /day) 0.4 - 3.3 g/m 7
offshore area (100-250 mg C/m 2 /day) 0.2 - 0.4 g/m ?

Based on this distributional pattern, we defined three subareas: the productive, equatorial
and transitional areas (Fig. 17.1).

Off Hawaii, Clarke (1973) made a seasonal study of the size composition, vertical dis-
tribution and abundance of myctophid species. Sampling was done in four seasons (September,
December, March and June) by IKMT-10 ft and a much larger midwater trawl in the - ca. 1000m
layer. The abundance of the dominant nine species was 0.32 g/m in wet weight and 0.55/m
in number. He attributed the lowest biomass, observed in June, to the fact that in most
species few adults were still alive then while the juveniles of the next generation had not
yet recruited to the population, indicating a short life span.

- 106 -

Maynard et al. (1975) reported the faunal composition, standing stock and vertical mi-
gration of mesopelagic animals off Hawaii, based on ten samples collected by IKMT-10 ft from
the 0-1 200 m layer in September-October 1972. The catches in day and night sampling were
not significantly different in the upper 1 000 m layer. Estimated densities in weight and
number are shown in Table 17.1. In total, the density of all species was estimated at 2.57
g/m 2 in wet weight and 5.1/m 2 in number. In light of the study of seasonal variation of
myctophid biomass by Clarke (1973), these values obtained in September and October may be
assumed to be intermediate between the maximum and minimum.

King and Iversen (1962) reported relative abundance of all micronekton as measured by
IKMT-10 ft catches in a wide range of the tropical Pacific (10N to 10S, 110W to 160W)
and the subtropical and subarctic Pacific (150W to 18Qo) .

Grandperrin and Rivaton (1966) reported faunal changes along the equator from 92W to
168W. Hartman and Clarke (1975) also reported a change in the species composition of myc-
tophids along 145W longitude between 12N and 330 ! S. These data on relative abundance and
faunal changes are also adopted here to define the subareas of different density.

Extensive larval surveys have been carried out in the eastern subtropical and tropical
Pacific as part of the CalCOFI and EASTROPAC I and II surveys (Ahlstrom, 1965, 1969, 1971,
1972; Ahlstrora and Counts, 1958). The results have consistently shown the dominance of
mesopelagic fish larvae in the oceanic area, although total biomass cannot yet be reliably
estimated from these data owing to lack of knowledge on the ecology of the various species,
including their fecundity, spawning season etc.

Some qualitative information is available from acoustic surveys. Chapman et al. (1975)
found a difference in the spectrum of column scattering strength between the north subtropi-
cal and the tropical regions of the eastern Pacific. This difference was also observed in
volume scattering measurements at 12 kHz (Johnson, 1976). The acoustically detected boundary
off the southern end of Baja California coincides with faunal changes in mesopelagic animals,
including fishes detected by other means, and is adopted here as the boundary between the
transitional area and the productive area defined earlier in this section (Fig. 17.1).

Synthesis of abundance estimates

Considering the distributional pattern of abundance together with the zoogeographical
zones and major water masses, the Eastern Central Pacific can be divided into five subareas
(Fig. 17.1): the. transitional area (small transitional area in the South Pacific tentative-
ly being included here), the Eastern North Pacific Central Water area, the Eastern South
Pacific Central Water area, the equatorial area and a productive area off Central and South
America.

The biomass value of 3.6 g/m 2 , obtained in the transitional area of the Northeast Pacif-
ic (FAO Fishing area 67), was taken for the transitional zone, since these areas are con-
tiguous and identical.

The Eastern North Pacific Central Water area is rather small and is probably affected
by an inflow of biomass from the more productive surrounding areas such as the transitional
and equatorial areas. Therefore, its mean biomass density is presumed to be higher than
those of the more extensive central water areas in the northwest and southeast Pacific. The
density of 2.57 0.81 g/m 2 obtained in September-October 1972 off Hawaii by Maynard et al.
(1975) is the most reliable value available at present, and is thought to be near the annual
mean, since Clarke (1973) found the maximum biomass of nine dominant myctophid species in
the same area in December and the minimum in June. But the sampling was done 10 - 25 km
off the west coast of Oahu in the Hawaiian Archipelago where the density is thought slightly
higher than the average in the open Eastern North Pacific Central Water owing to the effect
of the islands. Therefore a lower value, 2.0 g/m ? , was tentatively adopted for the calcula-
tion of biomass in the central witer of the eastern North Pacific.

- 107 -

Table 17.1

The standing stock of mescpelagic fishes off Hawaii
based on 1KMT-10 ft sampling in September -October 1972
(Maynard et a'L, 1975)

Mean biomass, grams wet weight per 100 m 2 ocean
surface. Standard deviation in parentheses

Day

Night

Group

- 1200 m

- 400 m

Myctophidae

65.71 (20.36)

0.19 (0.17)

69.85 (12.26)

Cyclothone

45.91 (11.26)

0.46 (0.75)

1.20 (2.92)

Other

Gonostomatidae

14.92 (8.48)

0.41 (0.49)

29.24 (25.22)

Sternoptychidae

25.05 (14.02)

0.39 (0.91)

7,45 (2.75)

Other Stomiatoidei

15.56 (12.53)

0.00

13.36 (14.73)

Anguilliformes

48.18 (43.65)

2.40 (3.04)

1.53 (1.63)

Misc. fishes

41 37 (40.16)

2.65 (0.93)

9,46 (6.35)

Caridea

50.49 (21-07)

0.15 (0.28

30.27 (13.11)

Penaeidea

31.59 '10.61)

0,13 (0.10)

22.71 (3,88)

Euphausiacea

18.52 (3.98)

0.86 (0.80)

12.28 (1.57)

Mysidacea

8.80 (9.76)

0.00

4.42 (3.45)

Misc. Crustacea

1.09 (1.92)

0.00

1.09 (0.78)

Cephalopoda

48.71 (47.46)

2.02 (2.02)

13.84 (16.28)

Tunicata

34.07 (42.52)

5.90 (3.50)

21.68 (15.00)

Cnidaria

40.86 (46.86)

10.50 (5.50)

11.34 (12.87)

Misc . invertebrates

3.38 (3.49)

6.61 (2.35)

1.39 (0.33)

Total micronekton

494.20 (99.30)

32.68 (6.58)

251.11 (53.46)

Total fishes

256.70 (81.30)

6.50 (2.99)

132.09 (39.33)

Total Crustacea

110.49 (36.15)

1.14 (1.06)

70.77 (12.30)

Cephalopoda
Other invertebrates

48.71 (47.46)
78.31 (53.50)

2.02 (2.02)
23.01 (8.34)

13.84 (16.28)
34.41 (21.93)

Zooplankton j,
Zooplankton -

48.12 (21.64)
471.95 (212.26)

15.24 (4.07)
149.45 (39.94)

49.20 (12.30)
482.57 (120.91)

- 1 Calculated assuming 7.7 m 2 net mouth, full 10-foot IKMT mouth.
^ Calculated assuming 0.785 m 2 net mouth, cod end mouth area.

- 103 -

Table 17.1 (Contd.)

Mean number of organisms per 100 m 2 ocean
surface. Standard deviation in parentheses

Day

Night

Group

- 1200 m

- 400 m

Myctophidae

108.13 (45.71)

1.02 (1.15)

80.72 (22.65)

Cyclothone

308.29 (103.34)

6.07 (8.84)

19.48 (39.01)

Other

Gonostomatidae

25.77 (3.70)

3.25 (3.84)

22.13 (8.56)

Sternoptychidae

22.97 (5.98)

0.42 (0.78)

4.36 (1.47)

Other Stomiatoidei

3.92 (2.04)

0.00

2.68 (1.70)

Anguilliformes

11.43 (4.46)

2.64 (1.04)

2.96 (2.23)

Misc. fishes

27.49 (7.64)

16.31 (5.68)

21.08 (5.83)

Caridea

40.18 (5.93)

1.70 (2.21)

27.52 (11.18)

Penaeidea

137.90 (34.99)

6.83 (3.74)

132.90 (21.69)

Euphausiacea

97.79 (28,16)

6.79 (5.77)

69.01 (11.50)

Mysidacea

9.89 (3.02)

0.00

8.73 (6.67)

Misc. Crustacea

4.08 (6.72)

0.00

1.88 (1.54)

Cephalopoda

8.44 (2.04)

3.60 (1.98)

5.95 (1.81)

Tunicata

28.05 (14.10)

50.26 (24.73)

25.01 (10.97)

Cnidaria

10.80 (9.41)

11.82 (2.49)

11.47 (8.81)

Misc. invertebrates

52.93 (25.02)

55.55 (7.73)

42.04 (10.03)

Total micronekton

898.07 (149.50)

166.27 (16.45)

477.94 (69.27)

Total fishes

508.00 (133.50)

29.71 (7.48)

153.41 (60.43)

Total Crustacea

289.84 (54.25)

15.32 (4.92)

240.04 (13.91)

Cepholopoda

8.44 (2.04)

3.60 (1.98)

5.95 (1.81)

Other invertebrates

91.78 (25.22)

117.63 (19.26)

78.52 (22.71)

No. organisms caught

12 037

1 576

5 136

No. tows

- 109 -

4ON

IIO C IOO 90 8O

E.V;: v| 1.0g/m 2 Central Water, S P

I'.".'.'."'.] 2.O " Central Water, N.P

^^j^j 3.0 " Equatorial area

^^^ 3.6 i Transitional area

illiillllli ^- " Productive area off Central America

17.1 The five subareas and values used for the
total hiomass estimation of micronektoni c
fish in the Eastern Central Pacific.

No measured value for the central waters in the eastern South Pacific is available at
present, so the lower value 1.0 g/m 2 from the southern part of the Western North Pacific
Central Water area was adopted for making a conservative estimate.

In the productive area east of the equatorial area, data from deep tows is lacking, but
the value of 4 g/m 2 is considered reasonable because densities of 2 - 3 g/m 2 have been ob-
served in the uppermost 100 m at night.

In the equatorial region, 3.0 g/m 2 was considered reasonable on the basis of the exten-
sive data on relative abundance from IKMT-10 ft catches (King and Iversen, 1962), primary
productivity and zooplankton volumes, although a reliable value is not yet available for
this area.

The biomass in each subarea is shown in Table 17.2. The total biomass in FAO Fishing
area 77 was estimated to be 129 x 10 6 tonnes, with species composition and productivity
probably varying considerably from area to area.

The need for more reliable biomass data on the equatorial area, the central water in
the South Pacific and the productive area should be stressed again.

Table 17.2

Estimated total biomass of the mesopelagic fish

in the Eastern Central Pacific based on measured

or assumed biomass densities by IKMT-10 ft

Subareas

Size of area
x ion m 2

Biomass
8/1 2

Total biomass
x 10 6 tonnes

Transitional area

3.6

Eastern North Pacific
Central Water area

158

2.0

Eastern South Pacific
Central Water area

143

1.0

Equatorial area

162

3.0

Productive area off
Central America

4.0

Total

129

- Ill -

Depth distribution

Paxton (1967a) analysed the distributional pattern of the lanternf ishes of the San
Pedro Basin,^California by successive IKMT-10 ft hauls to various depths. By day all the
fifteen species recorded were taken below 350 ra, with the centre of concentration between
450^and 700 m. Five species showed peak abundance below 650 m and ten species above 650 m
during the day. At night, four shallow species ascend the upper 10 m, five to 50 m, and
one to only 150 m. Of the five deep species, two were non-migrants and three migrated to
50 m depth. He pointed out temperature and light as the most important determinants of
vertical distribution, and suggested that the shallow thermocline might set the upper limits
of vertical migration of some species.

Barham (1966, 1970) observed myctophid fishes, including Triple tmto mcxicanuo and Steno-
brachiuB leueopsarus, off southern California from deep submersible vehicles. He reported
a dense concentration in the upper 100 m at night and in the 300 - 400 m layer during the
day, and related it to the deep scattering layers.

Maynard et al. (1975) studied the faunal composition, standing stock and diel vertical
migration of mesopelagic micronekton in Hawaiian waters by 9 deep-oblique tows (0-1 200 m)
and a 24-hour series of 14 consecutive shallow-oblique tows (0 - 400 m) . Of the total fish
biomass of 256.7 g/100 m 2 ocean surface in the - 1 200 m layer, 132.1 g/100 m 2 migrated
up to the upper 400 m. About 50% of the migratory biomass was composed of myctophids and
the other half mainly of gonostomatids, sternoptychidh and other stomiatoids.

In the Hawaiian waters, Clarke (l r >73) reported the vertical distribution of 47 mycto-
phid species. Most spe-.cies showed oniogenetir. cliff eiences in vertical distribution and mi-
gration, with the smaller fish tending to occur shallower than the adults. Large fractions
of the populations of three species, Notolychnus valdiviac^ LampanyctuCy niger and Triphotu-
rus nigresoens, appeared not to migrate in certain seasons.

Species compos i tion

Berry and Perkins (1966) surveyed the mesopelagic fishes of the California Current
area (20N - 40N) , collecting 198 samples between May 1961 and March 1963. More than 189
species and about 52 000 specimens were taken. The greatest number of species in a single
family was 40 in the Myctophidae. The most common species in the 198 samples are listed
below with the number of positive stations in parentheses.

Myctophidae:

Triphoturus mexicanus 76

Protomyotophwn croakem 66

Ceratoseoperus towns endi 66

Lampanyctus vitteri 63

Diaphus theta 61

Symbolophorus califomiense 55

Stenobraahius leucopGaruo 54

Tarletoribeania crenularis 45

Gonostomatidae :

Vinciguerria luoetia 25

Cyelothone signata 64

C. aoclinidens 49

Danophos oculatus 47

Sternoptychidae

Argyropelecus hawaiensis 54

A. papifious 49

A. intermedius 45

The most abundant mesopelagic species in a single tow was T. mexicanus, with about
3 000 specimens taken at one station and 1 857 at another. Other large single collections
were of C. townsendi (944), S. leueopsarus (735) and V. luoetia (537). These results in the
transitional area of the Eastern Central Pacific are believed to be reliable, since the sam-
pling coverage was wide in both area and season. A similar species composition has been
reported by Paxton (1967a) , It is notable that the species composition varies from north to
south, with the subarctic species being gradually replaced by the transitional, central
water and equatorial species (see Table 15*2).

Off Hawaii, Clarke (1973, 1974) reported some aspects of the ecology of myctophids and
stomiatoids based on the four series of samples collected quarterly from September 1970 to
June 1971 in the upper 1 000 m. The dominant species were as below, the figures to the
right showing number of specimens collected and their size range in mm.

Myctophidae

Ceratoscoperus warmingi 3911 11-79

Lampanyctus steiribeaki 2362 14-56

Triphoturus nigrescens 2 120 9-38

Lampanyctus niger 1946 12-135

Bolinichthys longipes 1458 11-56

Notolychnus vardiviae 1267 9-25

Benthoserna suborbitale 1 157 9-38

Diaphus sehmidti 823 9-47

Hygophwn poximum 696 12-51

H. reinhardti 413 12-48

Gonostomatidae

Vinciguerria nimbaria 2927 8-49

Cyolothone alba

C. pseudopallida

C. pallida

C. acclinidence

Gonostoma atlanticum 680 10 - 66

G. elongatum 1 346 10 - 218

Valenciennellus tripuno kulatus 600 10 - 32

The species of the genus Cyolothone in the above list were added on the basis of reports
by Mukhacheva (1964). This genus was not dealt with by Clarke (1974).

In the equatorial region, Hartman and Clarke (1975) analysed the distributional pattern
of myctophids collected in the upper 50 - 75 m at night along 145W longitude between 12N
and 330 f S and identified three faunal groups. The first group was distributed across the
entire transect, being most abundant at or just north of the equator, and included #. proxi-
mum> Symbolophorus evermanni, T. nigrescens, C. uarmingi and B. longipes. The second group
occurred only in the North Equatorial Current or the Counter Current, and included Diogenich-
thys latermatusj Diaphus garmanij Lampanyotus nobilis and L. omostigma. The third group
occurred only in the productive area at or just north of equator, and included Diaphus mala-
yanus* D. signatus^ Lampanyotus bubbsi.

- 113 -

Grandperrin and Rivaton (1966) pointed out four faunal zones along the equator between
92W and 162E, based on the analysis of 24 LKMT-5 ft samples collected at night in the upper
300 m* Each zone was marked at the time of the cruise by an 'endemic 1 fauna that could be
related to the variable depth of the Cromwell Current along the equator. Three of the four
zones fall within the Eastern Central Pacific.

All the findings mentioned above show that the Eastern Central Pacific takes in several
different zoogeographical areas which may be further divided into subareas. Species com-
position is believed to vary in each subarea.

Life history

Stenobraehius leucopsarus shows the same growth and reproduction patterns in Monterey
Bay, California, as it does off Oregon, although the spawning season is about 2-3 months
delayed in Monterey Bay (Fast, 1960; Smoker and Pearcy, 1970).

Clarke (1973) studied some aspects of the ecology of 47 myctophid species off Hawaii,
and estimated the spawning season of 23 species and sizes et maturity for 33 species. Most
myctophids studied were believed to have a one-year life cycle, so in many species the population
would be almost totally replaced with each new generation. A similar life history pattern
seems to be common in tropical species, especially the species less than about 50 mm as
adults.

- 115 -

18. SOUTHEAST PACIFIC

Information on abundance and depth distribution is limited in this region. Therefore,
biomass densities were mostly inferred by analogy from values measured in similar areas else-
where.

This area was divided into four subareas: the upwelling area off Peru and Chile, the
Eastern South Pacific Central Water area, the subantarctic area, and the transitional areas.

Abundance

Blackburn (1968) reported the abundance of microncktonic fish in the upper 95 m at night
as shown by 1.5 m square net samples. The area surveyed covers a wide area from the coast
to as far westward as 95W between 5S and 25S, providing useful information for the defini-
tion of subareas.

Cradock and Mead (1970) reported the inshore-offshore variation in abundance of midwater
fish (ml/hr collected by the IKMT-10 ft) along 34S latitude between 72W and 92W. The
higher biomass values, 100 - 600 ml/hr, were obtained in the inshore area east of 75W and
the values for the more offshore areas were below 100 ml/hr and not highly variable.

The results of larval surveys indicate that the larvae of one genus of the Gonostomati-
dae occur in greater total abundance *. ban those of the anchoveta (pers. com. Dr. R. Jordan,
IMARPE, Peru).

Synthesis of abundance specie. s

Based on the limited data available and the distribution of primary production and zoo-
plankton biomass, this area was divided into four subareas as shown in Fig. 18.1. Mean den-
sity in the upwelling area off Peru was assumed to be the same as in the highly pi oductive
area off Central America in the neighbouring Eastern Central Pacific. Density values for
the transitional, subantarctic and central water areas were inferred by analogy from the
values in other areas that are similar to these subareas in terms of primary production and
zooplankton abundance.

The biomass estimated for each subarea is presented in Table 18.1, with a total of 52 x
10 6 tonnes in the entire area. Estimates based on reliable field data are badly needed for
the Southeast Pacific, especially the upwelling and subantarctic areas.

Table 18.1

Estimated total biomass of mesopelagic fishes
in each subarea of the Southeast Pacific

Subareas

Size of subarea
m 2 x 10 n

Density
g/m 2

Stock
tonnes x 10 6

Upwelling area off Peru

4.0

Eastern South Pacific

1.3

Central Water

Transitional area

3.6

Subantarctic area
Total

4.5

- lib -

V^3 13 g/m 2 Eastern South Pacific Central Water

3.6 " Transitional area
^j 4.0 " Upwelling area off Peru

4.5 " Subantarctic area

IOOW 90

Fig. 18.1 The four subareas and values adopted for the total
biomass estimation of micronektonic fish biomass in
the Southeast Pacitic

Depth distribution

No quantitative information is available at present.

Bussing (1965) reported that fewer individuals and species were taken in the deeper
hauls than in hauls of less than 2 000 m in the Southeast Pacific. It is remarkable that
large numbers of individuals and species of fishes were collected in the oxygen minimum
zone (0.2 - 0.5 ml/) between 50 and 800 m and the greatest number of specimens (2 765) in
a single haul was presumably captured at 680 m which coincides with the oxygen minimum
layer at that locality.

Species composition

Extensive faunal studies of mesopelagic fishes were done by Bussing (1965) and Parin
et al. (1973) in the eastern South Pacific. According to their findings, the area is in-
habited by four groups of species, i.e. the equatorial water species, the central water
species, the subantarctic water species and the cosmopolitan species. The area off Peru
and Chile is a zone of zoogeographic transition where the fishes of all four groups occur.
The common myctophid and gonostomatid species in their samples are as follows:

Gonostotnatidae: Vinaigu&rria luaetia, Cyc'lothone. signatha , C. aeclinid&no.

Myctophidae:

Hygophum veirihardti, Dioyeniehthys laternatus > Symbolophorun boops,
S. evexinannij *tyctophwn cQAVolaternatwn 9 M. mtidulurn, Gonirhthya
tenuiaulus 3 Notolyc'hnus valdiviae, Triphoturus mexieanus* Lompan^^tu
aohirus, L. omo stigma* L. parvicauda.

Life history

No information is available.

- 117 -

19. SOUTHWEST PACIFIC

This area was divided into three subareas: the central, transitional and subantarctic
areas (Fig, 19,1). The subareas were defined on the basis of the distribution of primary
production, zooplankton abundance and water masses, and mesopelagic fish densities were
estimated on the basis of the observed relationship between environmental conditions and
density in other areas. Future estimates based on measured biomass density may well result
in appreciable corrections in this area.

Abundance

For the density in the eastern South Pacific Central Water area we adopted the value
of 1.3 g/m obtained in the northern part of the Western North Pacific Central Water area,
considering the rather high biomass density in the neighbouring transitional and subantarc-
tic areas. For convenience, the small productive area off northern New Zealand is included
in the subantarctic area. The value of 4.5 g/m 2 for the subantarctic was set at the same
level as in the eastern subarctic, and the value of 3.6 g/m 2 for the transitional area is
that measured in the transitional area in the Northeast Pacific. The estimated biomass in
each subarea is shown in Table 19.1. A total of 101 x 10 6 tonnes was estimated in this area.

According to Robertson (1977), the stock of Lampanyotodes hevtoria, the only myctophid
species commercially fished off southwest Africa, seems considerable off New Zealand . Future
abundance surveys of this species should be carried out.

Table 19.1

Estimated total biomass of mesopelagic fishes
in each subarea of the Southwest Pacific

Subareas

Size of subarea
m 2 x 10 n

Density
g/m 2

Stock
tonnes x 10 6

Eastern South Pacific

1.3

Central Water area

Transitional area

147

3.6

Subantarctic. area

4.5

Total

101

Depth distribution

No reliable information is available, but the vertical distributional pattern of bio-
mass is probably similar to that in similar areas elsewhere, since the species composition
partly overlaps that of neighbouring areas where the pattern is well known, for both migra-
tory and non-migratory species.

- 118 -

30 S

I50E

160 170 180 I70 P 160 150 140 130 120

.x'.iy/J I 3 g/m z Eastern South Pacific Central Water
Transitional area
Subantarctic area

Fig. 19.1 The three subarcas and values used for the total
biomass estimation in the Southwest Pacific:.

Species composition

Quantitative studies on the species composition have not yet been done in the South-
west Pacific, but according to the results of zoogeographical surveys (Andriyashev, 1962;
Bekker, 1965; Nafpaktitis and Paxton, 1968; Nafpaktitis and Nafpaktitis, 1969; Gorbunova,
1972; Mukhacheva, 1972, 1974; Wisner, 1976, etc.), the following species are believed to
be common in the area.

Myctophidae:

Prvtomyctophwn andersoni, P. nomani, P. bolini, P. Gubparallelwn,
Electrons aa^lsbergi^ '. subasper'a, DiogenivhthyB atlantiauti ,
Hygopkm hygomi, H. rcinhardti^ H. hanscni-,, Syn\bclopliorus boops^
Gonichthys bamer>i> Centrobranchus choepocephalus, Diaphus hudsoni,
Lampadeyw speauligera, L. d&a^ L. notialis, L. chavesi, Lampanyctus
intriearius* L. auetralis^ Lampanyctodes heatoriX) CaratoBaopelus
warmingi, GyrnnoaeopeluB piabilis.

Gonostomatidae: Cyalo thane peeudapallida, C. pallida, C. rnicrodon, Gonostoma bathy-
philum, G. longipinnis , Vinaiguerria attenuate.

Life history

The eggs of L. hectoris, reported from off New Zealand, are the first myctophid eggs
ever identified to species (Robertson, 1977). Future life history studies of this species
are expected, since at present L, hectoris is the only exploited species among the meso-
pelagic fishes. Life history studies of other species are lacking in this area.

- 119 -

20, THE ANTARCTIC

The Antarctic as defined here is the area south of the Antarctic Convergence, roughly
corresponding to 50S in most of the Atlantic and Indian Ocean sectors and to 60S in the
Pacific sector.

The^Antarctic region is one of the least known as far as mesopelagic fish are concerned,
since scientific work there has been seriously restricted by its remote location and severe
sea conditions, especially in winter. In view of the lack of data and the absence of any
well surveyed area analogous to the Antarctic, no estimation of stock size was attempted for
this review.

Only a brief description of the Antarctic mesopelagic: fish fauna will be given here,
based mainly on Andriyashev (1962). The waters around Antarctica are inhabited by the pelag-
ic fishes belonging to at least a score of families including the Myctophidae and Gonosto-
matidae. Other major families are the Bathylagidae, Paralepididac, Macruridae, Scopelarchi-
dae, Melamphaeidae, Trichiuridae, Notothenidae and Chaenichthyidae. In the northern area
near the Antarctic Convergence, there occur fishes of the families Sternoptychidae, Astro-
nestidae, Idiacanthidae, Stomiatidae, Notosudidae, Anotopteridae, Moridae, Oreosomatidae and
Cerathidae, which are common also in the subantarctic zone between the Antarctic Convergence
and the Subtropical Convergence. But it should be noted that the number of meso- and bathy-
pelagic families and their species diversity increase rapidly north of the Antarctic Conver-
gence,

In regard to the fishes of the Myctophidae and Gonostomatidae, the former exceed the
latter in the species diversity and probably in hiomasF. The main Antarctic gonostomatids
hitherto reported are the non-migratory species of the genus Cy do thorn (C. miarodon and
C. paeudopallida) . Therefore, their fishery potential seems low. The common myctophid
species are listed below. The Antarctic myctophid fauna is marked by the three diverse
genera Protomyatophwn* Flectporia and Cyrmon&opelun.

Myctophidae: Protiymyctophwn andersoni, P. bolini* Electrona antarctica, '. carlsbevgi,
GymnosQopeluG hr*aueri, G. nicholsi* G. piabilie.

Of these, E. ant,arctica and G. bmueri are thought to be the most abundant, based on
their frequency of occurrence in samples on zoogeographical surveys. It is noteworthy that
most of these species have a circumpolar distribution around the Antarctic continent.

No information on life history is available except Yefremenko ? s (1977) report of mycto-
phid eggs at the cleavage stage in depths of 200 - 500 m in the Scotia Sea (Ca. 55S, ca.
40W) in the Atlantic sector.

There remains much to be learned about the. mesopelagic fishes of the Antarctic region.
International cooperative surveys such as BIOMASS should produce extremely reliable informa-
tion.

- 121 -

21. DISCUSSION OF FISHERY POTENTIAL
21.1 Summary of abundance information

Some of the information on density and total biomass of mesopelagic fish is summarized
in Table 21.1. Although the data are sparse, and most of the gears used to obtain the avail-
able information obviously underestimate the biomass present, there is little doubt that the
density of mesopelagic fish in offshore areas is generally low, and probably mostly below
10 g/m . The density may, however, be considerably higher along continental shelves. In
the Arabian Sea there is good evidence of very large stocks of mesopelagic fish, and there
are also indications of high densities off West Africa and possibly on both sides of South
America. ^But there are also highly productive coastal areas where the densities of meso-
pelagic fish seem to be quite modest, e.g. off Mozambique and off the Seychelles.

Atlantic Ocean

In the Atlantic there are several interesting areas. Off western and southwestern
Africa there are indications of high densities of mesopelagic fish, but there is no reliable
data on the size of these neritic stocks. The fishery off southwestern Africa seems to in-
dicate that there is a seasonal variation in the abundance of LjmpanyatQdes heatorie near
shore, or at least in availability to the fishing gear. There also seem to be large fluc-
tuations from year to year.

Off other parts of West Arnica there is no information on seasonal or annual variation
in the density of mesopeiagic fishes.

Off southern Norway and west of Great Britain mesopelagic fish seem to occur in fairly
high densities, and they sometimes form aggregations vulnerable to fishing gears. In this
area too, the data on natural fluctuations are sparse.

Off central parts of South America there are indications of large mesopelagic fish
shoals, but more investigations are needed before any conclusions can be reached.

Indian Ocean

The Arabian Sea seems to hold the largest stock of mesopelagic fish known so far.
Stock size estimates for the eastern and northern part of the area have been obtained over
a period of several years, and 5 estimates for the area ranged between 60 and 150 million
tonnes. Other estimates covering only part of the area suggest a still wider range of vari-
ation. It is not known how much of the observed variation is caused by sampling error and
how much by real fluctuation in stock size.

The stock of slope water species seems considerable in the Bay of Bengal and off the
southern coast of Australia, although there remains much to be surveyed.

Pacific Ocean

Most of the FAO Fishing Areas in the Pacific include various water masses and zoogeo-
graphical zones. In this review, each area has been subdivided in accordance with relative
densities (Sections 14 to 19). All of the subareas are shown in Fig. 21.1 with the density
value (g wet weight/m 2 ) adopted for estimating total biomass.

In the North Pacific, the distributional pattern of mesopelagic fish, as so far
measured, conforms with the distribution of the major water masses, primary production and
zooplankton abundance. Faunal change of mesopelagic species also corresponds to this pat-
tern. These relationships were assumed to exist, and were used for estimating biomass in

Table 21.1
Summary of mesopelagic fish densities and stock sizes

Area

Main - ,
investig.

Dominant
species

Density
S/m 2

Biomass
tonnes x 10 6

N.E. Atlantic

Benthosema glaciate

0.1 - 2

Offshore N of 60N

Offshore S of 60N

Benthos ema glaciate

0.5 - 2

Neritic W of G.Britain

ANR

Mauroliaus muelleri

10 - 35

and Norway

ELS

Total

N.W. Atlantic

B&nthosema glaoiale

5-8

N of 60N

0.1 - 4.7

S of 60N

Ceyatoscopelus maderensis
Benthosema glaciate

0.1 - 1.7

Neritic S of Newfound-
land

Ceratoscopelus maderensis
Benthosema glaciate

10 - 60

Total

E.G. Atlantic

Cyolothone spp.
Argyropelecus hemigymnus

4-6

Offshore waters

Mauritanian upwelling

ANR/CT

Diaphus dwnerili

15 (-60)

Total

W.C. Atlantic

Notolychnus valdiviae

0.1

Gulf of Mexico

Caribbean

Diaphus dwnerili

Q..1

Oceanic Areas

MN
AR

(Dionenichthys atlanticus )
\Ceratoscopelus warmingi I

0.1 - 0.2
0.2 - 1

(Lepidophanes guentheri )

Total

S.E. Atlantic

MN *

Lampanyctodes hectoris

3 ?

2 ?

Coastal zone

Offshore areas

1.2

Total

See list of abbreviations at end of table.

Table 21.1 (Contd.)

- 123 -

Area

Main . ,
investig.

Dominant species

Density
S/ 2

Biomass
tonnes x 10 6

Northeast Pacific

Subarctic area

Stenobrachius 'leucopsarus

4.5

Diaphus theta

Tarletonbeania vrenuralis

Transitional area

Stenobrachius Icucopsarus

3.6

Diaphus theta

Protamyctophum rwoek^ri

Total

Western Central Pacific

Southeast Asian Seas

faaphus gamani

4.5

D. vegani

D. auborb'italis

Equatorial current

Diaphus gannani

2.6

system area

Western North ar.J South

Diaphus malayanus

1.0

Pacific Central Water

areas

Total

Eastern Central Pacific

Transitional area

TriphoturuG mexicanus

3.6

Protomyetophttm crockeri

CevatOQQOpelus townsend'i

Vinciguerria luoet-ia

Eastern North Pacific

CeratosGope'lus waimingi

2.0

Central Water area

Lampanyetus steiribecki

Triphoturus nigveaoens

Vinciguerrn-a nimbairia

Eastern South Pacific

1.0

Central Water area

Equatorial area

Hygophum proximum

3.0

DiaphuB garmani

Diogen-iahthys laternatus

Productive area off

4.0

Central America

Total

129

Table 21.1 (Contd.)

Area

Mai n< f
investig.

Dominant species

Density
g/m 2

Biomass
tonnes x 10 6

S.W. Atlantic

Coastal zone

Diaphus dwnerili

high ?

Offshore areas

0.5

0.3 - 1.8

Total

Mediterranean

Cyc'lothone braueri

0.1 - 1

Total

W. Indian Ocean

Arabian Sea West

ANR/CT

Benthosema ptevotum

8 - 220

100

11 East

ELS

Benthosema pterotum

Off Mozambique

ANR/CT

Diaphus spp.

1-30

Oceanic areas

Various

0.5

140

Total

257

Eastern Indian Ocean

Subarea I

Diaphus tuetkeni

4.7

Subarea II

D. splendidus

3.8

Subarea III

3.1

Subarea IV

1.8

Total

Northwest Pacific

Subarctic area

Stenobraohius nannochir

S* leucopsarus

6.5

Diaphus theta

Kuroshio system area

Diogenichthys atlantieus

Benthosema suborbitale

5.2

Diaphus kuroshio

Gonostoma graoile

Cyclone atraria

Vinciguerria nimbavia

- 125 -

Table 21.1 (Contd.)

Area

. Main . I/
investig,

Dominant species

Density

Biomass
tonnes x 10 6

Northwest Pacific (Contd.)

Cyclothone atraria
Gonostoma graeile

1.3

Western North Pacific
Central Water area

Diogenichthys atlanticus

Benthosema sulorbitale

Total

Southeast Pacific

4.0

Upwelling area off Peru

Transitional area

3.6

Eastern South Pacific
Central Water

1.3

Subantarctic area

4.5

Total

Southwest Pacific

1.3

Eastern South Pacific
Central Water

Transitional area

3.6

Subantarctic area

4.5

Total

101

Abbreviations

MN Micronekton nets

CT Commercial trawls

ELS Egg and larva surveys

ANR Acoustics, non-resonant frequencies

AR Acoustics, resonant frequencies

DO Direct observations from submissibles

X No quantitative field data; densities

estimated by analogy with similar

areas elsewhere

- 126 -

J 03

3 a;
r u

a>
en x!

CU 4-1

C
a;

Ct U^

.So

0)
(U

OJD
H

- 127 -

the South Pacific for which no reliable data are available at present. Although no zoogeo-
graphical data collected so far have contradicted these relationships, it should be stressed
that these estimates can only be tentative until reliable direct measurements are made.

All of the adopted density values are based on micronekton nets of various kinds, con-
verted to equivalent IKMT-10 ft values, with the exception of IKMT-6 ft values in the sub-
arctic eastern Pacific. Acoustic and egg and larva surveys arc still inadequate for estimat-
ing biomass in the Pacific.

^ ^ The most promising areas for future surveys, from the fisheries standpoint, can be iden-
tified as the productive area off Central and South America, the Southeast Asian Seas, and
the subarctic and subantarctic areas, mainly on the basis of high density in those areas
(Fig. 21.1).

In the subarctic area, a total stock of 31 x 10 tonnes was estimated. Most of the
stock is composed of oceanic species, some of which possibly form localized dense patches.
Therefore, future study should be directed to the schooling behaviour or patchy distribution
of the dominant species. Acoustic and egg and larva surveys may be also promising because
of the rather simple, abundant fauna in this region. Tn the subfmcarctic area the present
estimate should be interpreted as tentative; future estimates based on direct measurements
may well result in large corrections.

The Southeast Asian Seas, including the South China, Sulu, Celebes and Banda Seas, show
high biomass densities, compai Able to the subarctic rngion, and total biomass was estimated
as 14 x 10 B tonnes. Th : . s area is also rich in the larger myctophid species endemic to the
slope waters, since the continental aud insular slopes are well developed here (Table 21.3).
But most of the stock of large myctophids is not included in the present estimate, mainly
owing to their effective net avoidance (see Section 16) . Acoustic surveys are needed to
estimate the abundance of these large myr.tophids.

In the equatorial area off the coasts of Central and South America, the total biomass
was estimated as 21 x 10 tonnes, but scientific work is still insufficient to estimate the
total biomass reliably. Future abundance surveys should take into account the effects of
upwelling and the oxygen-minimum layer on the distribution of mesopelagic fishes.

Anong the other areas not yet mentioned, the transitional zone or the Kuroshio area
north of its axis is considerably productive, inhabited by endemic species and a promising
area for future study.

Finally, the total biomass estimated in this review should be interpreted only as a
value based on the data now available. Future refinements of estimates may well result in
substantial changes in several cases. But we believe that the true total biomass is within
three times the biomass estimated by the IKMT-10 ft data in the oceanic area.

21.2 Potential yield

A simple, approximate estimate of the maximum annual yield of an unfished population
can be derived from the equation Y max = 5 MB Q where M is natural mortality and B Q biomass
of the unfished stock (Gulland, 1971; Clark, 1978).

For most tropical mesopelagic fish the instantaneous mortality rate is supposed to be
higher than 2. Therefore, the annual yield can, according to the formula, be higher than
the mean standing stock. For temperate species the few estimates available suggest mortal-
ity rates between 0.7 and 2, and potential yield may therefore be about half the amount of
the standing stock.

These estimates must, however, be treated very carefully. It is not known how meso-
pelagic fish species will react to exploitation, but as they generally have a low fecundity
there may be a closer correlation between parent stock size and subsequent recruitment than

- 123 -

in most other fish stocks. The low fecundity may make them vulnerable to reproductive fail-
ure which may cause the stocks to collapse. Therefore, it is of vital importance that any
fishing for these species be carefully monitored to study any variation in growth, natural
mortality and recruitment.

21.3 Species of potential commercial interest

In Table 21.1 we listed the dominant species in various regions mainly based on the
micronekton net catches. But the species listed do not always correspond to the species of
potential commercial interest due to the limited catching ability of micronekton nets, es-
pecially with respect to large or surface migrating myctophid fishes. To offset these short-
comings, the species of potential commercial interest will be discussed here in relation to
their distributional pattern, regardless of their frequency in micronekton net catches.

In considering the possibility of harvesting mesopelagic species, several factors must
be taken into account. One is behaviour in relation to fishing techniques, e.g. schooling
behaviour, diel vertical migration, attraction to light. Other factors are related to fish-
ery planning and management such as potential yield and its variability. Finally, ways of
processing and marketing the catch are still to be developed.

The vertical distributional pattern is a very important factor for a potential fishery.
According to their vertical migrations, mesopelagic species can be broadly classified as
surface migrants, midwater migrants and non-migrants.

Surface migrants; Most of the myctophid species of the genera LoDeina^ Tarletonbeania^
Goniehthys* Centrobpanchus, Hygophum, Syribolophorus and Myetophw are known to concentrate
in the uppermost 10 m or less at sunset and return to the upper mesopelagic zone below 200m
at dawn. Species which may be considered as interesting members of this group are listed in
Table 21.2. At night they are collected in considerable quantities by surface tows made
from the side of a ship but they are rarely collected in tows made from the stern due to
scattering of fish in the ship's path. They are also lured by a floating ship's lights or
fish-luring lights and often dipnetted. During the daytime they are difficult to catch with
small trawls such as micronekton nets, probably because of visual net avoidance in the upper
mesopelagic zone. These traits suggest that the biomass of these species has probably been
greatly underestimated in surveys done with micronekton nets.

Their bioroass is also difficult to estimate by acoustic methods as they are above the
range of hull-mounted echosounders during the nighttime. As this group usually has well-
developed gas-filled swimbladders, they form a good acoustic target during the daytime.
Surveys by this method seem promising.

Midwater migrants: Species of this group do not occur at the surface at night, but
they form dense concentrations within the upper 100 m. The group can be divided into oceanic
and slope water species based on their horizontal distribution.

At present the slope water species are the most promising group since they are vulnerable
to existing fishing techniques or slight modifications of them. During the daytime some of
them stay just off the bottom (benthopelagic layer) over insular or continental shelves and
slopes, and can be caught in bottom trawls. At night they can be caught by commercial mid-
water trawls when they migrate to the upper 100 m. The only existing myctophid fishery, in
South Africa, catches a species of this group (Lampanyctue hectoris) . The important slope
water species to be surveyed are listed in Table 21.3.

In regard to oceanic species, the subarctic, antarctic and transitional areas are
thought to be more promising than the tropical zone because they have simpler faunas and
higher densities. The interesting species are Benthosema glaoiale in the North Atlantic and
Stenobrachius leucopsarus and Diaphus theta in the North Pacific. The antarctic myctophids,
Electrons, antarctica and Gymnosoopelus broueri are also of great interest.

- 129 -

Table 21.2

Coramon surface-migrating myctophid species in the
Atlantic, Indian, Pacific and Antarctic Oceans

Species

Centpobranchus brevirostris

C. choerooephalus

C. nigroooellatus

C. andreae

Gonichthys GOCGO

G. baimesi

Hygophwn hygomi

H. proximwn

H. reintiardti

H. hanseni

//. maeroahir

H. taaningi

Lowina interrupta

L. rara

Myctophum asperum

M. aupolaternatum

M. nitidulum

M. spinoswn

M. lychnobium

M. affine

M. puna to. turn

M. obtusirostre

M. phengodes

Symbolophorus boops

S. ealiforniensis

S. evermanni

S. rufinu.8

Tarletonbeania crenularis

Atlantic
Ocean

Indian
Ocean

Pacific
Ocean

4-
4-

4-
4-
4-
4-
4-
4-
4-

4-
4-

- 130 -

Table 21.3

Slope water species of potential commercial interest
in the Atlantic, Indian and Pacific Oceans

Species

Atlantic
Ocean

Indian
Ocean

Pacific
Ocean

Gonostomatidae

Mawpolicus muelleri

Myctophidae

Benthosema pterotwn

B. fibulatum

Diaphus watasei

D. ooeruleus

D. suborbitalis

D. burtoni *

D. umbrooulus *

Lampanyotodes hectoris

Notoseopelus japonicus **

* Reported only from the Southeast Asian Seas
** Endemic to the Oyashio-Kuroshio transitional area off Japan

Non-migrants; Most species of the genus Cyolothone and some of the genus Gonostoma
(Gonostomatidae) do not make diel vertical migrations, remaining in the mesopelagic zone.
The same might be true of some myctophid species of the genera Eleotrona and TaaningiahthyG.
Non-migrants do not form dense schools and show a rather even distribution in a certain
layer characteristic of the species. Frequency of occurrence and quantities caught have
been high in micronekton net samples due to their sluggish swimming behaviour. At present
the fishes of this group, from a fishery point of view, can be regarded as the least inter-
esting mesopelagic species because of their deep habitat and dispersed distribution. The
high wax content of most species of this group (Table 3.1) should also be recalled.

21.4 Present catches

At present there is little fishing for raesopelagic species. Off South Africa a mycto-
phid fish Lampanyetodes heotoris has been caught in the purse seine fishery directed mainly
toward the anchovy. The catches have fluctuated greatly (Table 21.4); the largest was
about 42 000 tonnes, taken in 1973 (Newman, 1977), The gear used is purse seines, generally
20 - 35 fathoms deep, i.e. with an effective fishing depth of about 30 - 50 m (Newman et al. 9
1978). The fish has been used for meal and oil production.

- 131 -

Table 21.4

Catches of lanternfish off South Africa

Data from FAO Yearbook of Fishery Statistics

for 1977 and Centurier-Harris (1974)

Year

Catch
tonnes

1969

1 134

1970

18 198

1971

2 600

1972

I 1 ) 200

1973

42 400

1974

316

1975

1976

132

1-J77

5 650

A questionnaire sent out by FAO shows that Soviet vessels catch myctophids off West
Africa (W. Fischer, pers. com.), but no further information on size of catch or species
composition is available.

Off southeast Australia mesopelagic fish (mainly Maurolicus melleri but also Lampanyc-
todes heotons) have been caught during experimental fishing. So far the catches have been
small, but the prospects have been described as promising (Anon, 1977; 1977a) .

Off Japan some DiaphuR species are caught and consumed locally but they are not marketed
(Kawaguchi and Shimizu, 1978).

- 133 -

22. NEEDS FOR FUTURE STUDIES

There are many important fields that nead further studies and only a few of them can
be treated here.

Those areas where there are indications of dense mesopelagic fish concentrations should
be further surveyed in order to (1) obtain more reliable stock size estimates, and (2) moni-
tor natural fluctuations in stock size. The most important areas in this context seem to be:

1. Arabian Sea

2. The Bay of Bengal

3. Northwest Africa

4. Southwest Africa

5. East coast of Central South America

6. Southeast Asian seas

7. Japan Sea

8. Transitional area between the Oyashio and Kuroshio

9. Subarctic area in the North Pacific

10. West coast of Central America and Peru

The subantarctic and antarctic regions and the waters around various sea-mounts remain
to be investigated.

Adequate knowledge of life history is restricted to only a few species and should be
acquired for other species, especi.ilJy dominant species. Further work is needed to evaluate
the daily growth ring for ageing mesopelagic fish. It it is proved reliable, more tropical
species should be aged for growth and mortality studies.

Difficulties in the identification of mesopelagic fishes have caused considerable
problems in community studies. Therefore, preparation of practical keys is required for
species identification worldwide. Egg and larva surveys have been inadequate mainly due
to the difficulties in identification, except in the California Current region. Further
work on egg and larva taxonomy is needed to prepare practical keys.

Studies should be carried out to trace any stock-recruitment relationship in the most
important mesopelagic fish species (e.g. BGnthosema ptepotwn> JJiaphus dwneri-li, Lampanyc-
todes heetoris and MauroHvus mualleiri,) . The data needed are long time-series of abundance
estimates and preferably also of egg/larva production. This study should be started while
the stocks are unexploited, and it should be given high priority if a commercial fishery is
initiated. Other aspects of reproductive biology, e.g. fecundity, spawning behaviour and
egg and larval survival, also need further research.

The role of mesopelagic fishes in the marine ecosystem is still poorly understood.
Studies on the feeding habits of important species should be carried out to find at which
trophic level they feed and to estimate their consumption rate. It is also important to
learn more about predation on mesopelagic fish. There is evidence that they may serve as
forage for various fishes, cephalopods and marine mammals, but quantitative data are very
few and at present it is not possible to account for what happens to the large mesopelagic
fish production.

Better gears to catch mesopelagic fish are needed, both for sampling and for fishery
purposes. Sampling gears must be quantitative and they should be designed to sample dis-
crete depth layers. For commercial fisheries priority should be given to gears which can
catch species coming up to the upper 100 m during the night. The South African fishery for
lanternfish has shown that purse-seines can be used, but for most species trawls are probably
better suited. Trials with small-meshed trawls, designed to catch krill, appear to be prom-
ising, but much research on design and operation techniques remains to be done. Some species
(e.g.* Benthosema pterotum) are also easily caught during the daytime, but this requires

- 134 -

advanced acoustical equipment to monitor the trawl depth relative to the fish concentrations.
To catch non-migrant species and others which do not form dense concentration, trawls which
can filter very large quantities of water are needed, or artificial means of concentrating
the fish must be found. Probably these problems will not be easily solved in the near future,

For stock size estimation acoustic methods should be further improved. It is very im-
portant to get target strength measurements of the most important mesopelagic fish species,
and it is also important to study further the relationship between resonant frequencies and
size and species of fish.

For a rational utilization of mesopelagic fish stocks it will also be necessary to do
research on the processing of the fish and on the marketing of the products. But these
questions and the economic aspects which have been dealt with by Wijkstrom (1978), fall out-
side the scope of this report.

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Full text of "A Review Of The World Resources Of Mesopelagic Fish Fao Fisheries Technical Paper No 193"

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